| LIBRARY OF CONGRESS, t 
I **<•/. QB45 I 

# *, 

{UNITED STATES OF AMERICA.} 



SHORT COURSE 



A8TKONOMT 



USE OF THE GLOBES. 



HENRY KIDDLE, A.M., 

SUPERINTENDENT OF SCHOOLS , NEW YORK, AUTHOR OF " NEW MaITCTAL OF THE 

Elements of Astronomy." 




NEW YORK: 
IVISON, BLAKEMAN, TAYLOR & COMPANY, 

138 & 140 Grand Street. 
CHICAGO: 1&3 & 135 STATE STREET. 

1871. 



Entered, according to Act of Congress, in the year 1870, by 

HENRY KIDDLE, 

In the Office of the Librarian of Congress, at Washington. 






Electrotyped by Smith & McDougal, S2 Beckman Street 



PREFACE. 



rTIHE design of this work is to supply a brief course of 
-*- lessons in astronomy for the use of young pupils, or 
of those whose time and opportunities do not permit a more 
exhaustive study of the subject. It is based on the authors 
* X ew Manual of the Elements of Astronomy," of which, in 
some respects, it is an abridgment ; but many parts of the 
work have been greatly simplified, and the arrangement of 
topics has been somewhat changed, so as to be adapted to a 
work of lower grade. 

The objective plan of instruction has been followed as far 
as it is applicable to the subject under treatment, the pupil's 
attention being constantly directed to the phenomena ad- 
dressed to his own observation, and the reasoning made 
to proceed directly from them. Section First will, it is 
believed, be found especially useful in the accomplishment 
of this object, by awakening, at the commencement, an 
interest in astronomical observation, as the basis of all sub- 
sequent study of the science. 

Throughout the work, the arrangement of the paragraphs 
is adapted to the topical method of recitation — so desirable, 
as far as is practicable, in every branch of study, in order to 
train the pupil in habits of connected and logical statement. 



IV PREFACE. 

Questions have been, however, appended at the foot of each 
page, in order to facilitate the more minute examination 
of the pupil on the text. 

The Astronomical Index will be found useful and con- 
venient in affording a brief summary of definitions, for final 
review. The Problems for the Globe have been placed in 
connection with those parts of the work to which they 
seemed most intimately to belong, and where they can best 
be studied with the view to illustrate more fully the prin- 
ciples laid down. 

The author hopes that, by presenting the fundamental 
principles and most interesting facts of astronomy in this 
simplified and condensed form, he may aid in inducing a 
more general study of this useful and sublime science in the 
public and private seminaries throughout the country, from 
which it is too often excluded, to make way for subjects of 
far less value, both in respect to educational discipline and 
practical information. 

New York, January, 1871. 



CONTENTS 



>♦» 

INTKODTTCTION. 

PAGE 

Mathematical Definitions 7 

SECTION I. 

GENEEAL APPEABANCES OF THE HEAVENLY BODIES. 

The Horizon — Rising and Setting of the Sun — Apparent Motions 
of the Stars — Apparent Motions of the Planets — The Moon — 
Comets and Meteors 15 

SECTION II. 

THE EAKTH. 

Its Form, Motions, Size, etc. — Day and Night — Twilight — The 
Seasons — The Celestial Sphere — Questions for Review — Prob- 
lems for the Griobe. 22 

SECTION III. 

THE SOLAE SYSTEM. 

The Planets — Magnitudes — Orbital Revolutions — Distances — 
Axial Rotations — Aspects — Apparent Motions 50 

SECTION IV. 

DESCRIPTION OF THE SUN AND PLANETS. 

The Sun — Zodiacal Light — Mercury — Venus — Mars — Jupiter — 
Saturn — Uranus — Neptune — The Minor Planets, or Asteroids 
—The Moon 75 



Vi CONTENTS. 

SECTION V. 

PAGE 
ECLIPSES 115 

SECTION VI. 
Tides 120 

SECTION VII. 
Comets 125 

SECTION VIII. 
Meteors or Shooting Stars 136 

SECTION IX. 
The Stars '. 142 

SECTION X. 
Nebulae 161 

SECTION XL 
Time 170 

SECTION XII. 
Refraction 1~ 5 

SECTION XIII. 
Parallax 178 

Index of Astronomical Terms 182 

Appendix 188 



INTRODUCTION, 



MATHEMATICAL DEFINITIONS. 

1. Extension, or magnitude, may be measured in 
three directions; namely, length, breadth, and thick- 
ness. These are therefore called the dimensions of 
extension. 

Length is the greatest dimension; Thickness, the shortest; 
Breadth, the other. 

2. A Line is that which is conceived to have only 
one dimension. 

Lines have no real existence independently of extension, or 
solidity. They are purely abstract or imaginary quantities: the 
marks called lines are only representatives of them. 

3* A Straight Line is a line that does not change 
its direction at any point. 

4. A Curve Line is one that changes its direction 
at every point. 

o. A Point is that which is conceived to have no 
dimensions, but only position. 
A point is represented by a dot ( . ). 

Questions.— 1. How may extension be measured ? Dimensions? 2. What is a 
line ? What is remarked of lines ? 3. What is a straight line ? 4. A curre line ? 
5. A point? 



8 IX TROD UCTIOX. 

6. A Surface is that which is conceived to have two 
dimensions — length and breadth. 

7. A Plane Surface, or Plane, is a surface with 
which, if a straight line coincide in two points, it will 
coincide in all. 

That is, a straight line cannot lie partly in a plane, and partly 
out of it ; and if applied to it in any direction, it will coincide 
with it throughout its whole extent. The term 'plane does not 
imply any limitation, or boundary, but signifies indefinite direc- 
tion, without change, both as to length and breadth. 

8. A plane bounded by lines is called a Plane 
Figure, 

9. A Circle is a plane figure bounded by a curve 

line every point of which is equally 
distant from a point within, called 
the centre. 

10. The curve line that bounds a 
circle is called the Circumference, 

11. The Diameter of a circle is a 
straight line drawn through its centre 

from one point of the circumference to another. 

12. The Radius is a straight line drawn from the 
centre to the circumference. 

13. An Arc is any part of the circumference. 

14. A Tangent is a line which touches the circum- 
ference in one point. 

6. What is a surface ? 7. A plane surface ? What is remarked of it ? 8. What is 
a plane figure ? 9. A circle? 10. The circumference ? 11. The diameter? 12. The 
radius ? 13. An arc ? 14 A tangent ? 




IN TB OB UCTION. 



15. A Semicircle is one-half of a circle ; a Quadrant 
is a quarter of a circle. 

16. The circumference of a circle is supposed to be 
divided into 360 degrees, each degree into 60 minutes, 
and each minute into 60 seconds. 

Degrees are marked ( ° ) ; minutes, ( ' ) ; and seconds, ( "). 

17. An Angle is the difference in 
direction of two straight lines that meet 
at a point, called the vertex. 

It is of the greatest importance that the stu- 
dent of Astronomy should form a clear idea of 
an angle, since nearly the whole of astronomical investigation is 
based upon it. The apparent distance of two objects from each 
other, as seen from a remote point of view, depends upon the 
difference of direction in which they are respectively viewed ; that 
is to say, the angle formed by the two lines conceived to be drawn 
from the objects, and meeting at the eye of the observer. This is 
called the angular distance of the objects, and, as will readily be 
understood, increases as the two objects depart from each other 
and from the general line of view. 

C -_A_ - E 



18. Hie Angle cf Vision, or Visual Angle, is the 

angle formed bylines drawn from two opposite points of 
a distant object, and meeting at the eye of the observer, 

15. What is a semicircle ? A quadrant ? 16. How is the circumference of a circle 
divided ? IT. What is an angle ? Angular distance of objects ? 

2 



10 INTRODUCTION. 

. It will be easily seen that, as tbe apparent size of a distant object 
depends upon the angle of vision under which it is viewed, it must 
diminish as the distance increases, and vice versa. 

Thus, the object A B is viewed under the angle A P B, which 
determines its apparent size in that position ; but when removed 
farther from the eye, as at C D, the angle of vision becomes C P D, 
an angle obviously smaller than A P B, and hence the object 
appears smaller. At E F, the object appears larger, because the 
visual angle E P F is larger. The farther the object is removed, 
the less the divergence of the lines which form the sides of the 
angle ; and the nearer the object is brought to the eye, the greater 
the divergence of the lines. 

19. An angle is measured by drawing a circle, with 
the vertex as a centre, and with any radius, and finding 
the number of degrees or parts of a degree included 
between the sides. 

gp- 20. A Right Angle is one that con- 

^^ tains 90 degrees, or one-quarter of the 

&> circumference. 



21* When one straight line meets another so as to 
form a right angle with it, it is said to be perpendicular. 

22. A straight line is said to be 
, £tv$- e * perpendicular to a circle when it 

passes, or would pass if prolonged, 
x0 . through the centre. 

23. An angle less than a right 
■angle is called an Acute Angle; one 

greater than a right angle is called an Obtuse Angle. 




18. What is the angle of vision ? Remark ? Illustrate hy diagram. 19. How is 
an angle measured? 20. What is a right angle ? 21. A perpendicular? 22. A per- 
pendicular to a circle ? 23. What is an acute angle ? An obtuse angle ? 



,IN TRODUCTION. 



U 




In the annexed diagram, the semi- 
circumference is used to measure all 
the angles having their vertices, or 
angular points, at C. Thus BCD, 
containing the arc B D, is an angle of 
45 ° ; B C E, an angle of 90° ; and B C F, J 
of 120°. The points A and B are at 
the angular distance of 180°, or two right angles from each 
other. 

24. A Triangle is a plane figure 
bounded by three sides. 

A triangle that contains a right angle is 
called a Right-angled Triangle. 

A triangle having equal sides is called an 
Equilateral Triangle. 

2o. Parallel Lines are those situ- 
ated in the same plane, and at the 
same distance from each other, at all 
points. 

Parallel lines may be either straight or 
curved. 

The circumferences of concentric circles, 
that is, circles drawn around the same centre, 
are parallel. 

28. An, Ellipse is a curve line, from 
any point of which if straight lines be 
drawn to two points within, called the 
foci, the sum of these lines will be 
same. 




Parallel Lines . 




always the 



24. What is a triangle ? A right-angled triangle ? 
25. What are parallel lines ? 26. What is an ellipse ? 



An equilateral triangle? 



12 



INTR OB UCTION. 




The curve line DBEG represents an ellipse, the sum of the two 
straight lines drawn to F and F, 
the foci, from the points A, B, 
and C, respectively, being always 
equal. This sum is equal to the 
longest diameter, D E. 

27. The longest diameter 
of an ellipse is called the 
Major Axis ; and the short- 
est diameter, the Minor 
Axis. 

In the diagram, D E is the major axis, and B G the minor axis. 

28. The distance from either of the foci to the centre 
of the ellipse is called the Eccentricity of the ellipse. 

It will be readily seen that the greater the eccentricity of an 
ellipse, the more elongated it is, and the more it differs from a 
circle; while, if the eccentricity is nothing, the two foci come 
together, and the ellipse becomes a circle. 

The distance from the extremity of the minor axis to either of 
the foci is always equal to one-half of the major axis. 

In the above diagram, F O is the eccentricity, and B F is equal 
to D O. The amount of eccentricity of any ellipse is ascertained 
by comparing it with one-half the major axis. Thus, in the dia- 
gram, O F being about one-half of O D, the eccentricity of the 
ellipse may be nearly expressed by .5. 

29. A Sphere, or Globe, is a round body every point 
of the surface of which is equally distant from a point 
within, called the centre. 

SO. A Hemisphere is a half of a globe. 



27. What is meant by major and minor axis? 28. Eccentricity? 
29. What is a sphere ? 30. A hemisphere ? 



INTR OD UCTION. 



U 




31. The Diameter of a sphere is a straight line 
drawn through the centre, and pnrr 
terminated both ways by the 
surface of the sphere. 

32. TJie Radius of a sphere 
is a straight line drawn from 
the centre to any point of the 
surface. 

33. Circles drawn on the sur- 
face of a sphere are either Great 
Circles or Small Circles 

34. Great Circles are those whose planes divide the 
sphere into equal parts. 

35. Small Circles are those whose planes divide the 
sphere into unequal parts. 

36. Tlie Poles cf a Circle 

are two opposite points on the 
surface of the sphere, equally 
distant from the circumference 
of the circle. 

The poles of a great circle are, of 
course, 90° distant from every point 
of its circumference. 

Two circles of the sphere are parallel when they are equally dis- 
tant from each other at every point. 

Two circles are perpendicular to each other when their planes 
are perpendicular, or at right angles with each other. 




PEKPENDICTTLAH PLAITES. 



31. What is the diameter of a sphere ? 32. The radius of a sphere ? 33. How 
are circles of the sphere divided ? 34. What are great circles ? 35. Small circles ? 
36. Poles of a circle ? 



u 



INTRODUCTIO N. 




37. The Plane of a Circle, or of any other figure, is 
the indefinite plane surface on which 
it rnay be conceived to be drawn. 

38> A Spheroid is a body resem- 
bling a sphere. 

3,9. There are two kinds of sphe- 
roids : Oblate and Prolate Spheroids. 

40, An Oblate Spheroid is a sphere 
flattened at two opposite points, called 
the poles. 

41. A Prolate Spheroid is a sphere 
extended at two opposite points. 

Thus, an orange is a kind of oblate 
spheroid ; and an egg, a kind of prolate 
spheroid. 




OBLATE SPHEROID. 



37. What is meant by the plane of a circle ? 38. What is a spheroid ? 30. How 
many kinds are there ? 40. What is an oblate spheroid ? 41. A prolate spheroid ? 



SECTION I. 

GENERAL APPEARANCES OF THE HEAVENLY BODIES. 

1. The Horizon. — A person standing upon an open 
plain, or on the deck of a vessel at sea, seems to be at 
the centre of a circle, the circumference of which bounds 
his view on all sides, both of the earth and sky; the 
latter rising above him like a blue dome — during the 
day, lit up by the sun ; during every clear night, span- 
gled all over with glittering stars, and sometimes illumi- 
nated by the silver radiance of the moon. At whatever 
point of the earth's surface the observer stands, he finds 
his view still bounded by a circle of the same size ; and 
he everywhere seems to see an equal portion of the earth 
and sky. The circle which bounds our view is called 
the horizon. 

2. Above this circle the sun ascends in the morning 
and descends below it in the evening; in the former 
case it is said to rise, and in the latter to set. No one 
will fail to observe that the sun rises and sets at nearly 
the same points of the horizon each morning and even- 
ing ; that it passes across the heavens from one side to 
the other, reaching its highest point at noon ; and that 
this point (called the i?oint of culmination) is higher 
during the summer than during the winter. 

1. What is meant by the horizon ? 2. What is said of the movements of the snn ? 
What is the point of culmination ? 



16 THE HEAVENLY BODIES. 

3. Cardinal Points of the Horizon, &c. — In order to 
be able to describe the apparent motions of the sun and 
other bodies, it is necessary to give names to certain 
fixed points of the horizon. Thus, the point over 
which the sun is at noon is called the south / the point 
diametrically opposite to it is the north; that situated 
midway between the north and south, near which the 
sun rises, is called the east ; and the point opposite to it, 
the west. Hence, if a person stand facing the north, the 
south will be behind him, the east on his right hand, and 
the west on his left. These points are called the cardi- 
nal points of the horizon. The intermediate points are 
northeast, southeast, northwest, and southwest. 

4. Rising and Setting of the Sun. — A careful obser- 
vation of the apparent motions of the sun will show that 
it does not, every day in the year, rise exactly at the 
east, or set at the west ; but that the points of its rising 
and setting vary, sometimes being to the north of east 
and west, and at other times to the south of them. 
Twice, however, during the year, namely, in the latter 
part of March and September, it will be seen to rise 
exactly at the east, and to set exactly at the west. 

5. Apparent Motions of the Stars. — By watching 
the stars, it will be discovered that they perform the 
same general movement as the sun ; that is, they pass 

3. What are the cardinal points of the horizon ? How are they determined ? 
What names are given to the intermediate points ? 4. What is observed in regard 
t j the rising and setting of the sun ? When does it rise exactly in the east and Bet 
in the west ? 5. What may be discovered by observing the stars ? What arcs do 
they appear to describe ? What is meant by the pole star ? What are circles of 
daily motion ? 



THE HEAVE XL Y BODIES. 17 

across the heavens from east to west. It will be seen, 
too, that the arcs which they may be conceived to describe 
are not all of equal size, those of some stars being small 
portions of a circle, others very large portions, while still 
others describe entire circles. Thus, if we look toward 
the south, we find that some stars rise in the southeast, 
pass over the heavens at a small height above the hori- 
zon, and set in the southwest ; but, if we look toward 
the north, we observe that some stars rise in the north- 
east, pass nearly all around the heavens, and set in the 
northwest, thus describing nearly a whole circle. Still 
other stars will be seen to pass entirely around, never 
rising or setting, but apparently describing circles round 
a star that seems to be motionless in the heavens, — 
the Pole Star. The circles which these bodies may 
be conceived to describe are called Circles of Daily 
Motion. 

6. It will also be observed, that, although these stars 
are thus constantly changing their places in respect to 
the horizon, each one making an entire circuit in the 
heavens every twenty-four hours, they do not change 
their relative positions. Thus, if three stars should be 
seen so situated as to form a triangle, the same stars 
would be seen, year after year, still forming the same 
figure, being motionless in respect to each other. On 
this account, these bodies have been called Fixed Stars, 
and they thus afford a standard by which we determine 
the motions of other bodies. 



6. What also may be observed ? Illustrate this. What r.re the stars called on 
this account ? What do they afford ? 

3 



18 THE HEAVENLY BODIES. 

7. Apparent Motions of the Planets. — Occasion- 
ally, bodies having the general appearance of stars are 
seen that do not keep the same relative position with the 
other stars, sometimes being near one star, sometimes 
another ; sometimes appearing to move toward the east, 
sometimes toward the west, or for a short time remain- 
ing stationary in the heavens, like the other stars. 
These bodies are called planets ; that is, wandering 
bodies, since they seem to move about among the 
stars. 

8. Every one has noticed a brilliant body of this kind 
that appears sometimes in the west just after the sun lias 
set, sparkling like a glittering gem in the sky. This is 
the planet Venus, which is familiarly known as the 
Evening Star. The same planet, after shining for some 




VENUS AS MORNING AND EVENING STAR. 



7. What other bodies are seen ? How do they appear to move ? What are thoy 
called ? 8. What is the Evening Star ? The Morning Star ? 



THE HEAVEXLY BODIES. 19 

months in the west, gradually approaches the sun and dis- 
appears ; hut soon afterward we find it still shining in 
all its splendor in the east just before the sun rises, when 
ir is called the Moisting Star. It is, however, easily 
recognized as the same planet — the beautiful planet 
Yenus. Thus it appears to move to and from the sun, 
departing only a short distance from it on one side or 
the other. 

9. There are few persons who have not also noticed 
another resplendent planet, which, instead of remaining 
with us only a few hours, in the evening or the morning, 
sometimes continues in view during the whole night, 
rising in the east just as the sun sets, and setting in the 
west just as the sun rises. This planet, like the others, 
appears to move among the stars in a general direction 
toward the east, departing from and returning to the 
same star in about twelve years. So that, if we see it in 
a certain part of the heavens at any particular time, we 
must wait twelve years before Ave can again see it in 
precisely • the same place. This splendid orb is called 
Jupiter. 

10. Another planet, which shines with a remarkably 
red, fiery color, takes only about two years to pass from 
one star to the same again. To this planet has been 
given the name Mars. Still another, called /Saturn, 
shines with a singularly steady light ; and although, as 
compared with the brightest of the stars, is somewhat 
dull in its aspect, is yet quite a conspicuous body. Its 

9. What other planet is described. ? What are its apparent movements ? 10. What 
is said of ilars ? Of Saturn ? How may planets he distinguished from stars ? 



20 THE HEAVENLY BODIES. 

wanderings are not so apparent as those of the other 
planets that have been named, since it takes nearly thirty 
years to pass from one star and return to it again. These 
are the most conspicuous of the wandering stars or 
planets ; but others may also be seen, although with diffi- 
culty. These will be referred to hereafter. Planets may 
generally be distinguished from the stars by their steady 
light — the latter being characterized by their twinkling 
or scintillation. 

11. The Moon — its Motions and Phases. — Of all 

the heavenly bodies, the moon is perhaps the most beau- 
tiful and interesting ; although, in consequence of seeing 
it so often, we are very apt to pay but little attention to 
the singular phenomena which it presents. At its first 
appearance in the west, just after sunset, it looks like a 
thread of light in the blue sky, which, as twilight disap- 
pears, seems to be the partially silver edge of a dark orb, 
which is faintly visible. From night to night the cres- 
cent expands as the moon seems to depart further from 
the sun, until it is seen in the south at sunset, and then 
presents half of an illuminated circle. This, too, grad- 
ually widens, and, in about a week, we see its full round 
form clamber up the eastern slope of the sky just as the 
sun sets below the western horizon. After this, it ri>es 
later and later every night until it appears in the east in 
the morning just before sunrise, presenting the same 
slender crescent of light which we saw in the west on its 
first coming into view ; and soon afterward it disappears 



11. What is said of the moon ? What appearances does it present ? What are 
its phases ? 



THE HE AVE XL Y BODIES. 21 

for a short time, to begin again its monthly career. 
These various changes of form presented by the moon 
are called its Phases. 

12. Comets and Meteors. — Occasionally, beside the 
sun, moon, stars, and planets, we see a very singular 
looking body in the sky, presenting a kind of misty or 
cloudy aspect, and accompanied by a long train of light. 
Such bodies are called Comets. Quite frequently, also, 
there is seen what appears like a star falling from the 
sky. It darts or shoots across the heavens with amazing 
rapidity, and disappears almost before we can see it. 
Such bodies are called Meteors. Some of them are of 
great size, shine with various colors, and pass through 
the air followed by a long and brilliant train, and some- 
times explode with a loud noise. 

13. All the phenomena connected with these bodies it 
is earnestly recommended to the young student to watch 
very carefully, so as to ascertain whether the statements 
here made are correct. By so doing, he will, in the only 
efficient way, lay the foundation for a clear and extensive 
knowledge of the most sublime of all sciences — the sci- 
ence which treats of the heavenly bodies — Astrokomy. 



12. What other bodies are occasionally seen ? Describe their appearance. 
13. What is recommended to the student ? What is astronomy? 



SECTION II. 

THE EARTH ITS FORM, MOTIONS, SIZE, ETC. 

14. The body on which Ave live, named the Earth, 
is evidently a sphere or globe in form, for the following 
reasons : 

1. The earth and sky always seem to meet in a circle, 
when the view is unobstructed ; 

2. The top of a distant object always appears above 
this circle before the lower parts ; as the sails of a sniff 
before its hull ; 

3. The elevation of the spectator causes this circle to 
sink, so as to show more of the earth's surface, and 
equally on all sides ; 

4. The heavenly bodies appear to move around the 
earth, some in large circles, some in small circles — one 
particular star in the heavens not appearing to have any 
motion at all ; 

5. Navigators are able to sail entirely around the 
earth cither in an eastward or a westward direction. 

15 That the earth is in motion is evident from the 
apparent movements of the heavenly bodies. The sun, 
planets, and stars all appear to pass round the earth once 

14. What is the shape of the earth? First proof? Second? Third? Fourth? 
Fifth ? 15. Wlnt proofs are there that it is in motion ? What is the direction of 
its rotation ? What is meant hy its axis ? 



THE EARTH. ZJ 

every twenty-four Lours. Xow, wliile it is incredible 
that all these bodies, evidently at different distances from 
the earth, should revolve around it in exactly the same 
time, the simple supposition that the earth itself turns on 
its axis once every day explains all the appearances which 
they present. Every child has observed that when he is 
traveling in a steamboat or a railroad car, all distant 
objects appear to be moving in the contrary direction ; 
thus, the apparent westward motion of the heavenly 
bodies indicates a real rotation of the earth in the con- 
trary direction. The straight line on which the earth 
may be conceived to turn is called its Axis. 

16. Latitude and Longitude. — Points are located on 
the surface of the earth by measuring their distances 
from certain established circles conceived to be drawn 
upon it. The position of thcs3 circles is determined by 
their relation to two fixed points, called the Poles. These 
points are the two extremities of the earth's axis — one 
being called the Worth Pole, and the other the South 
Pole. The two points where the earth's axis if extended 
would meet the heavens are called the Celestial Pole*. 
The position of one of these (the northern) is indicated 
by the pole star. 

17. The great circle exactly midway between the two 
poles is called the Equator. Its plane divides the earth 
into northern and southern hemispheres. The great cir- 

10. How are points on its surface located? How are the position of these circles 
determined ? What are the poles of the earth ? The Celestial Poles ? How is the 
northern ascertained ? IT. What is the equator 1 What are meridian circles ? 
Meridians ? now do they divide the earth ? 



THE EARTH. 




cles that pass through the poles are called Meridian 
_ .,. Circles ; the half of a meridian circle, 

Meridians 7 

extending from pole to pole, is called 
a Meridian. The plane cf any meri- 
dian circle must therefore divide the 
earth into eastern and western hemi- 
spheres. 

18. The position of a place on the 
surface of the earth is indicated by its distance from the 
equator and from some fixed meridian. The distance of a 
place, north or south, from the equator is called its Lati- 
tude • its distance, east or west, from some established 
meridian is called its Longitude. For this purpose the 
meridian of London or Greenwich is generally used ; but 
sometimes that of Washington or Paris. Such a meri- 
dian is called a First, or Prime Meridian. 

19. Latitude is reckoned on a meridian from the equa- 
tor to the poles ; longitude is reckoned from the prime 
meridian round to the opposite meri- 
dian. Small circles parallel to the 
equator are called Parallels of Lati- 
tude. It will be easily perceived that 
the poles have the greatest possible 
latitude, namely, 90° ; and that places 
situated under the meridian opposite 
the prime meridian, have the greatest longitude— namely, 
ISO , east or west. 




18. How is the position of a place indicated ? What is latitude ? Longitude ? 
What is a first or prime meridian? 19. How is latitude reckoned? Longitude? 
What are parallels of latitude? What points have the greatest latitude? The 
greatest longitude ? 



THE EARTH. 720 

20. Dimensions of the Earth. — The size of the earth 
is ascertained by measuring the length of a degree on 
the meridian of any place ; and then, as there are 360 
degrees in the circumference of a circle, 360 times that 
length will be the circumference of the earth. The 
average length of a degree is a little more than 69-J miles, 
and the circumference of the earth, about 24,877 miles. 
Its diameter is 7,912 miles. Careful observations have 
determined that the shape of the earth is not perfectly 
spherical, its diameter at the poles being about 26 miles 
less than at the equator. It is, therefore, a little flat- 
tened at the poles, its true form being that of an oblate 
spheroid. 

21. Yearly Motion of the Earth. — This great body 
is not only rotating on its axis, but is also revolving 
around the sun. This is indicated by the apparent mo- 
tions of the sun with respect to the stars, which it must 
be remembered are fixed points. The sun has an easterly 
movement among these bodies similar to that of the 
planets ; and the time that elapses from its leaving any 
star until it arrives at the same again is 365 J days. This 
motion of the sun we know is not real, but is occasioned 
by the revolution of the earth around the sun from west 
to east. 

22. Ecliptic, Equinoctial, etc. — The great circle in 
tie heavens in which the sun appears to revolve around 

23. How is the size of the earth ascertained ? What is the length of a degree on 
the meridian ? What is the circumference of the earth in miles ? Its diameter ? 
What is the exact shape of the earth ? 21. What indicates a revolution of the earth 
around the sun ? How does the sun appear to move ? What does this prove ? 
21. What is the ecliptic? The equinoctial? At what angle do these circles cross 
each other ? What is this angle called ? What does this prove ? 

4 



m 



TUB EARTH. 



the earth every year is called the Ecliptic. If we 
conceive the plane of the equator to be extended on all 
sides, it will cut the sphere of the heavens in a great 
circle, which is called the Equinoctial y and observation 



POLE OF ECLlP Tlc 




P °LE OF ECLIPTIC 



determines that the ecliptic crosses the equinoctial, 
making with it an angle of 23± degrees. This is called 
the Obliquity of the Ecliptic, and proves to us that the 
earth's axis is inclined to the plane of its orbit, that is, 
the path in which it may be conceived to revolve around 
the sun. 



THE EARTH. 



23. The two opposite points where the ecliptic crosses 
the equinoctial are called the Equinoctial Points, or 
Equinoxes. The one which the sun passes in March 
is called the Vernal Equinox/ and the other, which 
the sun passes in September, is called the Autumnal 
Equinox. 

24. The two opposite points of the ecliptic at which 
the sun is farthest from the equinoctial are called the 
Solstitial Points, or Solstices. The one north of the 
equinoctial is called the Summer Solstice / the one south 
of it, the Winter Solstice. 

25. Signs of the Ecliptic, Etc.— In order to indi- 
cate the progress of the sun in the ecliptic and to define 
its position at any time, that circle is divided into twelve 
equal portions called Signs, each division being marked 
by a particular sign. The following are the signs, with 
their names, and the day of the month on which the sun 
enters each : 



Spring 
Signs. 

Summer 
Signs. 



{ T 
f ss 

< a 

In 



Autumn , 
Signs. < 

Winter J ^ 
Signs. ) *~ 



Aries 

Taurus 

Gemlxi 

Caxcer 

Leo 

Virgo 

Llbra 

Scorpio 

Sagittarius 

Capricorxus 

Aquarius 

Pisces 



March 20. 
April 20. 
May 21. 
June 21. 
July 23. 
August 28. 
September 23. 
October 23. 
November 23. 
December 22. 
January 20. 
February 18. 



Yeexal Equixox. 



Summer Solstice. 



Autolxal Equtxox. 



Vvlxter Solstice. 



23. What are the equinoctial points ? What is the vernal equinox ? The autum- 
nal equinox ? 24. What are the solstitial points ? How are they distinguished ? 
25. How is the ecliptic divided? Name the signs. When does the sun enter each"? 



## BAY AXD NIGHT. 

23. The distance of the sun from the Yernal Equinox, 
reckoned on the ecliptic, is called its Longitude; but when 
the distance is reckoned on the equinoctial, it is called its 
Right Ascension. The distance of the sun at any time 
from the equinoctial north or south is called its Declined 
tion. It will be obvious that the greatest declination of 
the sun is 23^ degrees. The same terms are also applied 
to the distances of the stars and planets from these cir- 
cles and points. The distance of a heavenly body from 
the ecliptic is called its Latitude. Thus latitude on the 
earth is reckoned from the equator ; but celestial latitude 
is reckoned from the ecliptic, while terrestrial latitude 
corresponds to declination. 

DAY AXD X I G II T . 

27. The rotation of the earth on its axis causes the 
succession of day and night. As the earth turns, every 
place is brought alternately into the light and into the 
shade. That portion of the earth's surface which is 
turned toward the sun, so that its rays can shine upon 
it, has day ; and the part turned away from the sun, being 
in the shadow of the earth, must have night. 

28. At certain times in the year the days are equal to 
the nights, while at other times they are either much 

26. What is meant by the longitude of the gun ? Its right ascension ? Its decli- 
nation? What is the greatest declination of the sun? To what other bodies are 
these terms applied ? What is meant by the latitude of a heavenly body f What is 
the difference between celestial and terrestrial latitude ? To what does terrestrial 
latitude correspond ? 27. What causes day and night ? To what part of the earth 
is it day ? To what part night ? 28. Why are the days sometimes longer or shorter 
than the nights ? When do places have longer day than night ? When the reverse ? 
Illustrate by the diagram. 



D A T A NB XI GHT. 29 

longer or much shorter. This is caused by the obliquity 
of the ecliptic, in consequence of which the sun's declina- 
tion is constantly changing. When the sun is north of 
the equinoctial, all places in the northern hemisphere 
have longer day than night, and those in the southern 
hemisphere, longer night than day ; but when the sun is 
south of the equinoctial, this is reversed. 

In the diagram, let H H repre- 
sent the horizon, P P' the axis of 
the celestial sphere, E E' the equi- 
noctial; let also S be the sun in 
north declination, and S' in south 
declination. It will be obvious that 
as the earth turns, the sun at S will 
appear to move in a diurnal arc, 
as a S ~b. greater than the nocturnal 
arc a c~h ; and at S', the diurnal 
arc m S' n will be less than the noc- roNGEST DAT J» OTGnT . 

turnal arc m o n ; while at E, in the 

equinox, the circle of daily motion described by the sun will be 
divided equally by the horizon. 

29. At places situated under the equator, the heavenly 
bodies appear to describe circles perpendicular to the 
horizon, and these circles of daily motion are all bisected 
by the horizon, so that the diurnal arc of the sun is con- 
stantly equal to the nocturnal arc. The days are, there- 
fore, at places thus situated, always equal to the nights. 
It is also obvious that, at places under the equator, during 
one-half of the year, the sun is in the north when on the 
meridian, and during the other half in the south ; while 

29. What do the heavenly bodies appear to describe at places under the equator ? 
How does this affect the length of day and night ? When is the sun vertical ? 
What places can have a vertical sun ? 




30 DAY AND NIGHT. 

on the 20th of March and the 23d of September, it is 
exactly overhead, or vertical. Since the sun's declina- 
tion is never greater than 23 |°, no place whose latitude, 
either north or south, is beyond that limit, can have a 
vertical sun ; and all places within these limits must 
have a vertical sun twice every year ; that is, as the sun 
moves north or south, and on its return. 

30. The small circles parallel to the equinoctial at the 
limit of the sun's declination are called the Tropics • 
that at the northern solstice is called the Tropic of Can- 
cer, and that at the southern, the Tropic of Capricorn. 
These names are used because when the sun arrives at 
one of these circles it turns back and goes to the other — 
the word tropic meaning turning. 

31. At the north or south pole there must be constant 
day during the whole six months the sun is north or 
south of the equinoctial ; it being constant day at one 
pole while it is constant night at the other. For to a 
person standing at the pole the equinoctial coincides with 
the horizon, and therefore when the sun is north of the 
equinoctial it must be above the horizon of the north 
pole and below that of the south pole. It will also be 
obvious that there must be constant day and constant 
night alternately to all places situated within 23J de- 
grees from either pole, and that its length must depend 
upon the distance from the pole. 

[These facts can best be illustrated by means of a globe or a tel- 
lurium, which will show the different portions of the sphere and 
the relation of the circles of daily motion to the horizon.] 



30. What are the tropics ? How are they distinguished ? Why is the word tropic 
used? 31. What occurs at the poles? Why? What places can have constant day 
and constant night ? 



DAY A XD NIGHT. 



31 



32. The two small circles par- 
allel to tlie equator, and 23 J de- 
grees from the poles, arc called 
Polar Circles. The one round 
the north pole is called the Arc- 
tic Circle ; and the one round the 
south pole, the Antarctic Circle. 
They serve to mark the limit at 
which constant day and constant 
night can occur. 



13- Pole 




S.Pole 
POLAE CIRCLES. 



33. When the sun is at either of the 
places in the same hemi- 
sphere with it have their 
longest day and shortest 
night; and those in the 
other, their shortest day 
and longest night. For 
Avhen the sun's declination 
is greatest, its meridian, 
altitude, and diurnal arc 
must be greatest, and its 
nocturnal arc least. 

34. When the sun is at 
either of the equinoxes, the 
days and nights are equal 
to each other in every part 
of the world. For, since 
the horizon of every place 




32. What are the polar circles? How are they distinguished? What do they 
mark? 33. What occurs when the sun is at either of the solstices ? Why? 34. What 
occurs when the sun is at either cf the equinoxes ? Why ? 



82 TWILIGHT. 

bisects the equinoctial, the sun's diurnal arc must every- 
where be equal to its nocturnal arc. 



twilight. 

25. When the sun is a short distance below the hori- 
zon, its rays fall on the upper portions of the atmos- 
phere, which, like a mirror, reflect them upon the eartli, 
and thus produce that faint light which is called twUighft 
The morning twilight is generally called the dawn. 




Let A B C represent three places on the earth, and A H", B II', 
C H, their horizons respectively. Suppose S to represent the sun, 
a little below the horizon, its rays passing through the atmosphere 
in S C H" ; at A, no portion of the visible atmosphere is illumi- 
nated, and consequently there is no twilight ; at B, the part W <j II 
is illuminated, and at 0, H" C IT : twilight is produced at each of 
these points. 

36. The duration of twilight varies greatly at different 
parts of the eartli; it is shortest at the equator, and in- 
creases toward the poles. Xear the polar circles and 
within them, there h constant twilight during a part of 
each year. 

3."5. How is twilight caused ? :! ;. How docs its duration vary f Remark ? 



TWILIGHT 



At the equator, the duration is 111. l~in. ; at the poles, there 
are two twilights during the year, each lasting about 50 days. 
This long twilight diminishes very much the time of total darkness 
at the poles ; for the sun is below the horizon six months, equal to 
180 days, and deducting 100 days of twilight, there remain only 80 
days, or less than three months, of actual night. 

37. Twilight does not coasc until the sun is about 1S C 
"below the horizon. 

If the earth's atmosphere were more extensive than it is, the 
twilight would of course be longer, since the sun would not cease 
to illuminate the higher portions of the atmosphere until more than 
18° below the horizon ; and if the atmosphere were less extensive, 
the reverse of this would be the case. Knowing, therefore, the de- 
pression of the sun (18°) requisite for the cessation of twilight, we 
can calculate the extent or height of the atmosphere. 

38. If the circles of daily motion were at all places 
equally inclined to the horizon, the duration of twilight 
would everywhere be the same ; since the earth would 
always have to turn 
the same amount to 
bring the sun 18° be- 
low the horizon ; but 
the more oblique the 
circles are, the farther 
the earth has to turn, 
and hence the twilight 
is longer the nearer Ave 
go to the poles. 

Let the large circle rep- 
resent the celestial sphere, 




DURATION OF TWILIGHT. 



37. W T hen does twilight cease ? Eemark ? 3S. Why does the duration of twilight 
vary ? Illustrate by the diagram. 

5 



n 



REASONS. 



e the earth in the centre; P II the altitude of the pole in one 
position of the sphere, and P' H its altitude in one less oblique ; 
E E and E' E' the equinoctial in each, and, of course, the direction 
of the circles of daily motion. In the more oblique sphere, 
that is, at the place in the more northern latitude, the celestial 
sphere, or which is the same thing, the earth, would have to 
turn a distance on the diurnal circle, equal to e a, to bring the sun 
18° below the horizon; while in the other position, the sun would 
reach the same point of depression when the sphere had turned 
only e 5. Thus we see the nearer the perpendicular the diurnal 
circles are, the shorter the twilight ; while the more oblique they 
are, the longer the twilight. 

THE SEASONS. 

39. The seasons are canned by the inclination of the 
earth's axis to the plane of its orbit. For the amount of 
heat received at any place from the sun depends upon 
the direction of its rays. In summer the rays are less 
oblique than in winter, and consequently the heat is 
greater; and this is still further augmented by the 
greater length of the day. 




- s 



STmVKB AND WINTER KAYS. 



In the annexed diagram, the effect of perpendicular and oblique 
rays is illustrated. It will be observed that the same quantity of 



33. How are the seasons caused ? Why '? Why is the heat greater in summer 
than in winter? 



THE SEASONS. 35 

rays that covers the north polar circle, when they are nearly per- 
pendicular, covers the whole space from the antarctic circle to the 
equator when they are oblique. 

40. The four seasons, Spring, Summer, Autumn, and 
Winter, are marked and limited by the arrival of the 




THE SEASONS. 



sun at the vernal equinox, northern solstice, autumnal 
equinox, and winter solstice, respectively. They are 

40. How are the four seasons marked and limited ? Why are they regular ? 



36 THE SEASONS. 

regular — that is, always tli3 same from year to year, 
because the axis always points in the same direction, oi 
remains parallel to itself. 

41. When the sun is at either of the solstices, summer 
is produced in that hemisphere which is turned toward 
the sun, and winter in the other, the rays falling directly 
on the former, and obliquely on the latter. AVhen tho 
sun is at either of the equinoxes, the earth's axis leans 
sidewise to it, and the rays are direct at the equator, and 
equally oblique on both sides of it. Consequently, there 
is neither summer nor winter; but spring in that hemi- 
sphere which the sun is about to enter, and autumn in 
that which it has left. 

[The above statements will be understood by an inspection of the 
diagram on page 35.] 

42. Those parts of the earth at which the sun may be 
vertical must have the greatest heat, and those parts at 
which there may be constant night must have the 
greatest cold ; while the parts between them must have 
a degree of heat and cold not so extreme as either. 

Hence the earth's surface has been 
divided into five portions, called 
Zones, the boundaries of which are 
the tropics and polar circles. 

43. These zones are called the 
Torrid, North Temperate, Sout/i 
Temjwrate, North Frigid ar.d 

41. What occurs when the sun is at either of the solstices ? When it is at either 
of the equinoxes ? Where is it spring? Autumn? 42 At what places must 1 he 
heat he greatest ? The cold ? How is the earth's surface divided ? 43. What 
names are given to the zones '? State the situation of each. 




THE CELESTIAL SPUEEE. 37 

South Frigid zones. The Torrid Zone includes the 
space between the tropics ; the Temperate Zones are 
between the tropics and polar circles ; and the Frigid 
Zones are within the polar circles. 

T II E CELESTIAL S P n E Ti E . 

44. By the Celestial Sphere is meant the concave 
sphere of the heavens, in which the heavenly bodies 
appear to be placed, the observer being at the centre 
within, and looking upward. The principal circle of 
this sphere is the horizon, which may be defined as the 
circle that separates the visible part of the heavens from 
the invisible. It is either Sensible or Dat tonal. 

45. The Sensible Horizon is the circle which bounds 
our view ; and its plane touches the earth at the place 
where the observer stands. The Iiational Horizon is 
below the sensible horizon, and parallel to it ; and its 
plane passes through the centre of the earth. Owing to 
the great distance of the heavenly bodies from the earth, 
these two circles very nearly coincide in the heavens. 

46. When, however, the observer stands at an eleva- 
tion above the surface of the earth, the sensible horizon 
sinks below the rational horizon, and more than one-half 
of the heavens becomes visible. This is called the Dip 
of the Horizon. 



44. What is the celestial sphere ? What is the principal circle of this sphere ? 
How may it he defined ? How distinguished ? 45. What is the sensible horizon ? 
The rational horizon ? How do they differ ? 43. What is meant hy the dip of the 
horizon ? How is it caused ? Illustrate hy the diagram. 




88 THE CELESTIAL SPHERE. 

This will be understood by studying the following diagram anc 
explanation : Let the small circle, whose centre is E, represent the 

earth, the portion of the larg( 
circle V Z V a part of the ce 
lestial sphere, and P the j>oint 
or place, of the spectator. Ther 
the tangent S P S will repre- 
sent the plane of the sensible 
horizon, and S Z S the visible 
heavens. Conceive the observei 
to stand above the surface at II : 
the tangents H V and H V will 
pensible and rationax hoeizon. then, at their points of contact, 

D and D, limit the visible part of the earth's surface, and nt their 
extremities, V and V, the visible heavens. S V or S V will be, oi 
course, the dip of the horizon. At the point P, the visible part ol 
the heavens is less than the invisible ; but at so great an elevation 
as II P (represented as about 1,000 miles), the visible part would 
be much greater than the invisible, and a large part of the earth's 
surface, denoted by the arc D D, would come into view. The dip, 
however, at any attainable height is very small, and only an incon- 
siderable portion of the earth's surface can ever be seen. The line 
Ii R represents the plane of a great circle, which divides the 
celestial sphere into equal parts, j>assing through the centre of the 
earth, and situated at a distance from the plane of the seusible 
horizon equal to the semi-diameter of the earth, or nearly 4,000 
miles. 

47. The poles of the horizon are called the Zenith and 
Nadir. The zenith is the point directly overhead ; the 
nadir is the point opposite to the zenith, and directly 
under onr feet One is, therefore, the pole of the 
visible, or upper hemisphere ; and the other, the pole of 



47. What arc the poles of the horizon ? What is the zenith ? The nadir ? What 
is the distance of each from the horizon ? 



THE CELESTIAL SPHERE. 



the invisible, or lower. Each is obviously 90 degrees 
from the horizon, 

48. Circles conceived to pass through the zenith and 
nadir, perpendicular to the horizon, are called Vertical 
Circle*. That which passes through the east and west 
points of the horizon is called the Prime Vertical. That 
which passes through the north and south points of the 
horizon also passes through the poles of the earth, being 
the Meridian of the Place,— -the circle which the sun 
crosses at noon. 

49. The distance of a body east or west from the 
meridian is called its Azimuth y the distance of a body 
above the horizon is called its Altitude. The altitude 
and azimuth of a body serve to indicate its exact position 

the visible part of the 
•elestial sphere. 

In the diagram, let X E S W 
'epresent the rational horizon ; 
he circle passing through X S, 
meridian ; and that passing 
hrough E W. the prime ver^i- 
:al; then, if A be the position of 
he sun at rising, A E will be its 
mplitude and A X its azimuth. 

hile its altitude will be 0°. ^adir 



Zenith 




QUESTIONS FOE EXEECISE. 

1. What is the latitude of the north pole ? 

2. What is the latitude of a place under the equator ? 



-1-3. What are vertical circles ? What is the prime vertical ? The meridian of the 
ace? -n. What is azimuth? Altitude ? What do they indicate ? 



JfO PROBLEMS FOR THE GLOBE. 

3. The latitude of New York being 40| degrees, what is its dis- 
tance from the north pole \ From the south pole \ 

4. What is the latitude of a place under the Tropic of Cancer I 
Under the Tropic of Capricorn ? 

5. What under the Arctic Circle ? Under the Antarctic Circle i 
G. What is the greatest latitude of a place ? 

7. What is the greatest altitude of a heavenly body ? 

8. Where is the altitude greatest ? Where is it least ? 

0. At what points is the declination of the sun greatest ? 

10. At what points is its declination least ? 

11. What is the right ascension of the sun in the frst degree of 
Cancer ? What is its longitude ? 

12. AVhat right ascension and declination has it in the first 
degree of Capricorn \ 

1 3. What in the vernal equinox ? In the autumnal equinox ? 

14. What is the longitude and latitude of the sun in the summer 
solstice ? In the winter solstice ? 

PROBLEMS FOR THE TERRESTRIAL GLOBE. 
PROBLEM I. 

To find the latitude and longitude of a place : Bring 
the given place to the graduated side of the brass merid- 
ian [the circle of brass that encompasses the globe], 
which is numbered from the equator to the poles : and| 
the degree of the meridian over the place will be tho. 
latitude ; and the degree of the equator under the me- 
ridian, east or west of the prime meridian, will be the 
longitude. 

Verify the following ~by the globe: 

LAT. 

LONDON, . . 51i° N. I 

Paris, . . 49° K 
Washixgton, 39° N. 
Cincinnati, 39° N. : 



LONG. 




LAT. 


LONG. 


0°. 


C. Good Hope 


34° S. 


lb£°E. 


2i°E. 


Berlin, . . 


52£° N. 


13l°E. 


77° W. 


Madras, . . 


13° N. 


80° E. 


84i° W. 


Santiago, 


32^° S. ; 


7C£° W. 



PROBLEMS FOB THE GLOBE. jj.1 

PROBLEM II. 

The latitude and longitude of a place being given , to 
find the place: Find the degree of longitude on the 
equator, bring it to the brass meridian, and under the 
given degree of latitude, on the meridian, will be the 
place required. 

EXAMPLES. 

1. What place is in lat. 30° N., and long. 90° W. ? Ans. New Orleans. 

2. What place " " 421° K, ht 71° W. ? Ans. Boston. 

3. What place " " 40f°K, " 74° W. ? Ana. New York. 

PROBLEM III. 

To find the difference of latitude or longitude between 
any two 'places : Find the latitude or longitude of both 
places ; if on the same side of the equator or meridian, 
subtract one from the other ; if on different sides, add 
them ; the result will be the answer required. 

EXAMPLES. 

Find the difference of latitude and longitude of 

1. London and Naples. Ana. Lat. 10-|-°, long. 14£-°. 

2. New York and San Francisco. Ans. Lat. 3°, long. 58-|-°. 

3. Stockholm and Rio Janeiro. Ans. Lat. 82°, long. 61°. 

Difference of Longitude causes Difference of Time. — Since the 
earth turns toward the east, any place east of another place must 
have later time, because it is sooner carried, by the motion of the 
earth, under the sun ; and as an entire rotation, or 360°, is per- 
formed in 24 hours, 15° of longitude must be equivalent to one 
hour of time. Thus, London is 74° east of New York ; and, conse- 
quently, when it is noon at New York, it is 5 o'clock in the after- 
noon at London, the sun having passed the meridian five hours 

earlier. 

6 



J$ PROBLEMS FOR THE GLOBE. 

Difference of Longitude may be converted into Difference of 
Time, by multiplying the degrees and minutes by 4 ; the formei 
of which will then be minutes of time, and the latter, seconds. 
For since T V the number of degrees is equal to the number of 
hours, -ff or 4 times, the degrees must be equal to the minutes ; 
and, for the same reason, 4 times the minutes of space must be 
equal to seconds of time. 

To convert Difference of Time into Difference of Longitude, 
reduce the hours to minutes, and divide by 4. For since 15 times 
the hours are equal to the degrees, -^ of 15, or £, the minutes must 
be equal to the degrees. 

PROBLEM IV. 
To find all the places that have the same latitude as 
any given place : Bring the given place to the brass 
meridian, and observe its latitude ; turn the globe round, 
and all places that pass under the same degree of the 
meridian will be those required. 

EXAMPLES. 

What places have the same, or nearly the same, latitude as 

1. Madrid ? Am. Minorca, Naples, Constantinople, Kokand, Salt 

Lake City, Pittsburgh, New York. 

2. Havana? Ans. Muscat, Calcutta, Canton, C. St. Lucas, Ma- 

zatlan. 

PROBLEM V. 

To find the places that have the same longitude as 
any given place : Bring the given place to the graduated 
side of the brass meridian, and all places under the 
meridian will be those required. 

EXAMPLE. 

What places have the same, or nearly the same, longitude as 
London ? Am. Havre, Bordeaux, Valencia, Oran, Gulf of Guinea. 



PROBLEMS FOB THE GLOBE. J/3 

PROBLEM VI. 

A time and place being given, to find what c? clock it 
is at any other place : Bring the place at which the time 
is given to the brass meridian, set the index to the given 
time, and turn the globe till the other place comes to the 
meridian, and the index will point to the time required 

Note. — If the place be east of the given place, turn the globe 
westward; if west, turn it eastward. 

This problem can be performed without the globe by finding the 
difference of longitude. 

EXAMPLES. 

1. "When it is noon at New York, what o'clock is it at London ? 

Am. 5 o'clock P. M. (nearly). 

2. AVhen it is 10 o'clock A. M. at St. Petersburg, what o'clock is 

it at the City of Mexico ? Am. 1 hour 20 min. A. M. 

3. When it is 9 o'clock P. M. at Rome, what o'clock is it at San 

Francisco ? Am. Noon. 

PROBLEM VII. 
To find the distance betioeen any tvjo places : Lay the 
graduated edge of the quadrant over both places, so that 
the division marked may be on one of them ; and the 
number of degrees between them, reduced to miles, will 
be the distance required. 

Note. — If geographic miles are required, multiply the degrees 
by 60 ; if statute miles, by 69i 

EXAMPLES. 

Find the distance in geographic and statute miles oetween 

1. North Cape and Cape Matapan. Am. 2,100 geog. miles ; 

2,418f statute miles. 

2. Rio Janeiro and Cape Farewell. Am. 4,980 geog. miles ; 

5,736f- statute miles. 



44 PROBLEMS FOR THE GLOBE. 

PROBLEM VIII. 

To find the sun's longitude for any given day : Look 
for the given day of the month on the wooden horizon, 
and the sign and degree corresponding to it, in the circle 
of signs, will be the sun's place in the ecliptic ; find this 
place on the ecliptic, and the number of degrees between 
it and the first point of Aries, counting toward the east, 
will be the sun's longitude. 

EXAMPLES. 

1. What is the longitude of the sun June 21st ? Arts. 90°. 

2. What is it February 22d ? Am. 334^°. 

3. What is it May 10th ? Ans. 50°. 

PROBLEM IX. 

To find the right ascension of the sun : Bring the 
sun's place in the ecliptic to the edge of the brass merid- 
ian ; and the degree of the equinoctial over it, reckoning 
from the first degree of Aries, toward the east, will be 
the right ascension. 

EXAMPLES. 

1. What is the right ascension of the sun October 18th ? Ans. 203£°. 

2. What is it May 2d \ Am. 42°. 

PROBLEM X. 

To find the declination of the sun: Bring the sun's 
place in the ecliptic to the edge of the brass meridian ; 
and the degree of the meridian over it, reckoning from 
the equator, will be the declination. The declination 
may also be found by bringing the given day of the 
month as marked on the analemnia to the meridian. 



PROBLEMS FOR THE GLOBE. Jfi 

EXAMPLES. 

1. What is the declination of the sun June 21st? Ans. 23|° N. 

2. What is its declination Jan. 27th ? Ans. 18±-° S. 

3. What is it April 16th ? Ans. 10° N. 

PEOBLEM XI. 

To find what places leave a vertical sun on any day in 
the year : Find the sun's declination, and note the de- 
gree on the brass meridian ; then turn the globe around, 
and all places that pass under that degree will be those 
required. 

EXAMPLES. 

1. What places have a vertical sun March 20th ? Ans. All places 

under the Equator. 

2. To what places is the sun vertical December 22d ? Ans. To all 

places under the Tropic of Capricorn. 

3. To what places is the sun vertical May 1st ? Ans. To all in lati- 

tude 16° N. 

PROBLEM XII. 

To find the meridian altitude of the sun for any day 
of the year, at any place : Make the elevation of the 
north or south pole above the wooden horizon equal to 
the latitude, so that the wooden horizon may represent 
the horizon of that place ; bring the sun's place in the 
ecliptic to the brass meridian, and the number of degrees 
on the meridian from the horizon to the sun's place will 
be the meridian altitude. 

EXAMPLES. 

1. Find the sun's meridian altitude at New Yoke:, June 21. Ans. 73°. 

2. At London, Jan. 27th. Ans. 20°. 

3. Rio de Janelro, September 23d. Ans. 67°. 



Jf6 PROBLEMS FOR THE GLOBE. 

PROBLEM XIII. 

To find the amplitude of the sun at anyplace, and for 
any day in the year : Proceed as in Problem XII. ; then 
bring the sun's place to the eastern or Avestern edge of 
the horizon, and the number of degrees on the horizon 
from the east or west point will be the amplitude. 

examples. 

1. Find the sun's amplitude at London, June 21st. Ans. 39f ° N. 

2. At Quito, September 23d. Ans. 0°. 

3. At Philadelphia, July 16th. Ans. 28° N. 

PROBLEM XIV. 

To find the surfs altitude and azimuth at any place, 
for any day in the year, and any hour of the day : Pro- 
ceed as in Problem XII. ; then set the index to twelve, 
and turn the globe eastward or westward, according as the 
time is before or after noon, until the index points to the 
given hour. Then, for a vertical, screw the quadrant of 
altitude over the zenith, and bring its graduated edge to 
the sun's place in the ecliptic ; the number of degrees on 
the quadrant from the sun's place to the horizon will be 
the altitude, and the number of degrees on the horizon, 
from the meridian to the edge of the quadrant, will be 
the azimuth. 

EXAMPLES. 

1. Find the sun's altitude and azimuth at New York, May 10th, 

9 o'clock A. M. Ans. Altitude, 45^° ; azimuth, 72£° E. 

2. At London, May 1st, 10 o'clock A. M. Ans. Altitude, 47° ; 

azimuth, 44° E. 

3. At London, March 20th, ^ o'clock P. M. Ans. Altitude 22° ; 

azimuth, 59° W. 



PROBLEMS FOR THE GLOBE. £7 

PROBLEM XV. 

To find on what two days of the year the sun is ver- 
tical at any place in the Torrid Zone : Turn the globe 
around, and observe what two points of the ecliptic pass 
under the degree of the brass meridian corresponding to 
the latitude of the place ; and the days opposite these 
points in the circle of signs will be those required. 

EXAMPLES. 
On what two days of the year is the sun vertical at 

1. Bombay ? Ans. May loth and July 29th. 

2. Bahia I Ans. Oct. 28th and Feb. 14th. 

• PROBLEM XVI. 

To find the time of the surfs rising and setting r , and 
the length of the day, at any place, and on any day in 
the year : Elevate the pole as many degrees as are equal 
to the latitude of the place, find the sun's place, bring it 
to the meridian, and set the index to twelve. Then turn 
the globe till the sun's place is brought to the eastern 
edge of the horizon, and the index will show the time of 
the sun's rising ; bring it to the western edge, and the 
index will show the time of the sun's setting. Double 
the time of its setting will be the length of the day ; and 
double the time of its rising, the length of the night. 

ISote. — The globe, of course, only shows this approximatively. 
A correction would also be required for refraction (see section XII.). 

EXAMPLES. 
At what time does the sun rise and set, and what is the length of the 
day and night, 

1. At London, July 17th ? Ans. Sun rises at 4, and sets at 8 ; 
length of day, 16 hours ; night, 8 hours. 



48 PROBLEMS FOB THE GLOBE. 

2. At New York, May 25th ? Ans. Sun rises at 4f , and sets at 
7|- ; length of day, 14£ hours ; night, 6£ hours. 

PROBLEM XVII. 

To find the length of the longest and shortest days and 
nights at any place not within either of the polar circle* : 
Find, by the preceding problem, the length of the. day 
and night at the time of the northern solstice, if the place 
be north of the equator, and at the time of the southern 
solstice, if it be south of the equator ; and this will be 
the longest day and shortest night. The longest day is 
equal to the longest night, and the shortest day to the 
shortest night. 

EXAMPLES. 
Wliat is the length of the longest and the shortest day 

1. At New York ? Ans. Longest day, 14 hours 50 min. ; short- 

est day, 9 hours 4 min. 

2. At Berlin ? Ans. Longest, 16^ hours ; shortest, 7-i- hours. 

PROBLEM XVIII. 

To find the beginning, end, and duration of constant 
day at any place 'within either of the polar circles : 
Take a degree of declination on the brass meridian equal 
to the polar distance of the place, then on turning the 
globe around, the two points on the ecliptic which pass 
under that degree will be the places of the sun at the 
beginning and end of constant day. Find the day of the 
month corresponding to each, and it will be the times 
required. The interval between these dates will be the 
duration of constant day. 

Constant night is equal to constant day at a place situated under 
the corresponding parallel in the ether hemisphere. Hence, to find 



PROBLEMS FOB THE GLOBE. 1$ 

the duration of constant night at a place in north latitude, find the 
length of constant day at a place having the same number of 
degrees of south latitude. 

EXAMPLES . 

Find the beginning, end, and duration of constant day and night at 

1. North Cape. Ans. Constant day begins May 14th, ends 

July 30th ; duration 77 days. Constant night 
begins November 25th, ends January 27th ; 
duration, 73 days. 

2. XoRTH Pole. Ans. Constant day begins March 20th, ends 

September 23d ; duration, 187 days. Constant 
night begins September 23d, ends March 20th ; 
duration, 178 days. 

PROBLEM XIX. 
To find the duration of twilight at any place not 
within either of the polar circles : Elevate the pole equal 
to the latitude, find the sun's place, bring it to the west- 
em edge of the horizon, and note the time shown by the 
index. Then screw the quadrant over the place, and 
bring its graduated edge to the sun's place ; turn the 
globe till the sun's place is shown by the quadrant to be 
18° below the horizon, and the time passed over by the 
index will be the duration of twilight. 

EXAMPLES. 

1. What is the duration of tvrilight at Loitbon, September 23d ? 

Ans. 2 hours. 

2. What is it at Dresden, April 19th ? Ans. 2 hours, 15 min. 



SECTION III. 

THE SOLAR SYSTEM. 

50. By means of the apparent motions of the planets, 
it is plainly seen that they revolve around the sun from 
west to east, receiving their light from that splendid 
luminary. The telescope reveals other bodies that re- 
volve around the planets, and are carried with them 
around the sun ; these are also called planets. All these 
bodies being dark except when illuminated by the sun, 
are called opaque bodies ; while the sun, giving light of 
itself, is called a luminous bod//. 

51. There are, therefore, two kinds of planets, namely, 
those that revolve around the sun only — called Primarm 
Planets, and those that revolve around the primaries and 
with them around the sun — called Secondary P hind*, or 
Satellite*. Of the latter the moon is an example ; sinc^j 
as is proved by its apparent motions and ] b is 
revolves around the earth every month, and accompanies 
the latter in its annual motions around the sun. 

52. Comets also revolve around the sun, but differ 
from planets not only in their appearance but in the 
direction of their motion, and the shape of their orbits. 



50. What do the apparent motions of the planet* indicate ? What does the tele- 
scope reveal ? What are opaque bodies ? Why is the sun called a luminous body ? 
51. IIow many kinds of planets are there? What example is given? 52. IIow do 
comets differ from planets ? What is the orbit of a body ? 



.~>2 THE SOL AR SYS TEM. 

For while the planets revolve from west to east, and in 
orbits nearly circular, comets sometime? 1 evolve from east 
to west, and in very elongated orbits. 

By the orbit of a body is meant the line which it may be 
conceived to describe in revolving around some other body. 

53. It will thus be seen that the apparent motions of 
the sun, planets, and stars are explained by supposing, 
1. That the earth is exactly or nearly a sphere ; 2. That 
it turns on its axis once every twenty-four hours ; 3. That 
the earth and all the other planets revolve around the 
sun ; and, 4. That the stars are situated at an immense 
distance from the sun and planets, in the regions of 
space, — a distance so vast that their movements witli 
respect to the earth and to each other cannot generally 
be discerned. 

The sun with all its attendant bodies constitute the 
Solar System. 

This arrangement of the sun in the centre with the planets 
revolving around it, is sometimes called the Copernican System, from 
Nicholas Copernicus, who, in 1543, revived the doctrine taught by 
Pythagoras, a Greek philosopher, more than 2,000 years previously, 
that the sun is the central body, and that the earth and planets 
revolve around it. 

Previous to Copernicus the general belief for more than two 
thousand years had been, that the earth is the centre of the universe, 
and that all the other bodies revolve around it in the following 
order : the moon, then the sun and planets, in their order, and 
then the stars. This system is very ancient, but being advocated 
and illustrated by Ptolemy, an eminent astronomer, who flourished 

53. How are the apparent motions of sun, planet?, and stars explained ? What 
constitutes the solar system ? What is remarked of it ? 



THE PLANETS. 53 

at Alexandria, in Egypt, about 140 A. D., it was subsequently 
called the Ptolemaic System. 

The doctrine of Copernicus, on its first promulgation in 1543, 
received little support, being generally rejected as visionary and 
absurd ; but the invention of the telescope, in 1610, and the dis- 
coveries made by means of it, by Galileo and others, afforded 
abundant evidence of its truth. 

THE PLANETS. 

54. There are eight large primary planets in the Solar 
System, besides a great number of smaller ones, called 
Minor Planets , or Asteroids. The names of the larger 
planets are Mercury, Venus, the Earth, Mars, Jupiter, 
Saturn, Uranus, and Neptune. Of these, Mercury and 
Venus revolve within the earth's orbit, and are on this 
account called Inferior Planets. Mars, Jupiter, Saturn, 
Uranus, and Neptune, revolving beyond the earth's orbit, 
are called Superior Planets. 

55. The fact that Mercury and Venus are inferior 
planets is shown by their never appearing but at a short 
distance from the sun. As already stated (Art. 8), 
Venus is never seen except as a morning or an even- 
ing star, and Mercury keeps always so near the sun 
that it is rarely seen at all. When it is visible, 
however, it shines with a very brilliant light in conse- 
quence of being so near the sun, twinkling like a fixed 
star. The other planets are known to be superior be- 
cause the j are seen at all distances from the sun, being 

54. How many primary planets are there ? What are their names ? Which are 
inferior ? Which are superior ? 55. How are Mercury and Venus known to he 
inferior ? How are the other planets known to be superior ? 



5Jf. 31 A G XI TUBES OF THE 

sometimes seen at one point of the horizon while the sun 
is rising or setting at the opposite point. 

56. The Minor Planets are very small bodies which 
revolve around the sun between the orbits of Mars and 
Jupiter. The number discovered is 112. Eighteen satel- 
lites are known to exist in the solar system ; of which 
the earth has one, — the moon ; Jupiter has four ; Saturn, 
eight; Uranus, four; and Neptune, one. 

57. All the planets in the solar system revolve in their 
orbits from, west to east, except tin satellites of Uranus 
and the satellite of Neptune, which revolve from east to 
west. As we view the planets, that is, looking towards 
the south (since the latitude of New York is 40|° north), 
the motion of the planets is from right to left, or the 
reverse direction of the hands of a clock. 

MAGNITUDE OF THE SUN AND PLANETS. 

58. The sun is by far the largest body in the solar sys- 
tem, being more than 500 times as large as all the planets 
taken together. Its diameter is a little more than 850,001 
miles. The following are the diameters of the large 
primary planets in miles : 



1. 


Jupiter, . 


. 85,000. 


5. 


Earth, . 


. 7,912. 


2. 


Saturn, . 


. 70,000. 


G. 


Venus, . 


. 7,500. 


3. 


Neptune, 


. 37,000. 


7. 


Mars, . 


. 4,300. 


4. 


Uranus, . 


. 33,000. 


8. 


Mercury, 


. 3,000. 



56. What are the minor planets ? What is their numher ? How many satellites 
are there ? How are they distributed ? 57. How do the planets revolve ? In what 
direction is this motion as we view them ? 5S. What is the magnitude of the sun ? 
What is the diameter of each of the primary planets ? 



SUN AND PLANETS. Go 

59. The first four of these planets, being very much, 
larger than the remaining four, are called Major Planets ; 
the others, being in the vicinity of the earth, are called 
Terrestrial Planets. 

A clear idea of the comparative size of the sun and planets may 
be obtained by conceiving the sun to be a globe two feet in diam- 
eter. Mercury and Mars would then be of the size of pepper-corns ; 
the Earth and Venus of the size of peas ; Jupiter and Saturn, as large 
as oranges ; and Neptune and Uranus, as large as full-sized plums. 

69. The diameter of the moon is 2,160 miles ; the four 
satellites of Jupiter, excepting one, are somewhat larger 
than the moon ; and the eight satellites of Saturn, ex- 
cepting one, are considerably smaller than the moon. 
Those of Uranus and JSeptunc also are probably smaller. 

61. The volume of a body, by which is meant the 
amount of space 
which it occupies, 
depends upon its 
length, breadth, and 
thickness. Hence, 
the comparative vol- 
umes of spheres are 
as the cubes of their 
diameters. Thus, as 
the sun's diameter 
s ten times as great 
as that of Jupiter, 
the volume of the 

COMPARATIVE SIZE OF SVNT AND JTJPITEIJ. 




53. Which are called major planets? Which terrestrial planets? Why? Ulus- 
tration ? 60. What is the size of the moon and of the other satellites ? 61. What 

meant by volume ? On what does it depend ? How are the volumes of bodies- 
compared? 



06 ORBITAL REVOLUTION 

m 

former is 1000 (10 x 10 x 10) times as great as that of 
the latter. 

62. Two bodies may be equal in volume, but contain 
very different quantities of matter, owing to the different 
degrees of compactness of their substance. Thus, a piece 
of cork equal in bulk, or volume, to a piece of lead, evi- 
dently contains a very much less quantity of matter. 
The quantity of matter which a body contains is called 
its Mass ; the degree of compactness of its substance is 
called its Density. 

The mass of a body depends upon its volume and density 
taken conjointly. Thus, if the volumes are as 2 to 3, and their 
densities, as 1 to 5, their masses will be as 1 x 2 to 3 x 5. or as 
2 to 15 ; that is to say, the mass of the second will be 7£ times as 
great as the first. To find the comparative density, therefore, of 
two bodies when the volume and mass of each are known, divide 
the relative mass by the relative volume. 

63. The Terrestrial Planets are very much more dense 
than the Major Planets, — the average of the former 
being five times that of the latter. Mercury, the most 
dense of all the planets, is more than six times as dense 
as water. The density of the earth is nearly 5^ times 
that of water ; the density of the sun is only about one- 
fourth the earth's, being about 1 \ that of water. 

ORBITAL REVOLUTIONS OF THE PLANETS. 

64. No portion of matter can set itself in motion ; nor, 
when in motion, can it stop itself. Whatever sets a body 
in motion, or stops it when in motion, is called For<-r. 

62. What is meant by the mass of a body? Its density ? Illustration? 63. What 
is said of the density of the terrestrial planets ? 64. What is force ? 



OF THE PLANETS. 57 

65. A body when acted upon by a single force moves 
in a straight line ; and will continue to move in the same 
direction and with the same velocity, until acted upon 
by some other force. When a force acts once and then 
ceases to act, it is called an impulsive force ; when it 
acts constantly, it is called a continuous force. 

Thus, the muscular force of the hand in throwing a stone, or 
the force of gunpowder in firing a ball from a cannon, is impulsive ; 
the force of gravity by which a body falls to the ground is con- 
tinuous. 

66. When a body is impelled by two forces in different 
but not opposite directions, it moves in a straight line 
between them. This line is the diagonal of a parallelo- 
gram of which the lines that represent the direction and 
quantity of the forces are the sides, aod is called the 
resultant of the two forces. 

Thus, let A B represent 
the line over which the 
body A would pass in a cer- 
tain time under the influ- 
ence of one force, and A C, 
the line over which it would 
pass in the same time, if 
acted upon by another 
force ; then under the simul- 
taneous action of both forces, it will pass over the line A D in 
the same time, and continue to move in this line until acted upon 
by some third force. 

67. When one of the two forces is a continuous force, 



65. Distinguish between impulsive and continuous force. 66. How is a body 
affected by two forces ? Illustrate by the diagram. 67. When does the body move 
in a curve line ? What may this curve be ? What are the forces then called ? 




58 ORBITAL REVOLUTIONS 

the body is drawn, at every point, from the straight line, 
and, consequently, moves in a curve line; and tle^e two 
forces may be so related to each other that the body will 
move around the centre of the continuous force in a cir- 
cle or ellipse. In that case, the continuous force is called 
the centripetal force • and the impulsive force, the cen- 
trifugal force. 

68. The centripetal force is that by which the body 
tends to approach the centre, or point around which it is 
revolving ; the centrifugal force is that by which the 
body tends to fly off from the orbit in which it is 
revolving. 

69. The centripetal force which acts upon the primary 
planets is the attraction of the sun ; that which acts 
upon the secondary planets is the attraction exerted by 
their respective primaries. This arises from the general 
law, that all oodles attract each other in direct propari 
tion to their mass, and inversely as the square of their 
distance. 

That is, a body containing twice the quantity of matter of another 
body exerts twice the force ; but, at twice the distance, would 
exert only one-fourth the force. This law is called the Law of Uni- 
versal Gravitation ; it was discovered by Sir Isaac Newton in 1665. 

70. The centrifugal force must arise from an impulse 
given to the planets when they commenced their mo- 
tions ; since, without such an impulse, they would have 

68. What is the centripetal force ? The centrifugal force ? 69. What acts as a 
centripetal force upon the planets? What is the general law? Illustration? 
70. From what must the centrifugal force arise? What would result if it should be 
destroyed ? What would result from the suspension of the centripetal force ? 



OF THE PLAXETS. 59 

simply moved toward the sun and have been soon incor- 
porated with it. And if the centrifugal force were now 
destroyed, the planets would all move in straight lines 
to the sun ; while, if the attraction of the sun were sus- 
pended, they would move off into space in tangent lines 
to their orbits. 

Let A B represent the amount 
of the centripetal, and A C that 
of the centrifugal force, for a 
given time ; then completing the 
parallelogram, and drawing the 
diagonal A D, we find the point 
which the body when acted on 
by both forces will reach in that 
time. E, F, and G may be shown 
in a similar way to be the points 
reached by the body at the end of | 
successive periods of time of an 
equal length ; and thus, if the forces acted by impulses, the body 
would describe the broken line formed by the diagonals of the 
parallelograms ; but as the force of gravitation is a continuous 
force, the revolving body describes a curve, which may either be a 
circle or an ellipse. 

71. The three most important truths pertaining to 
planetary motion were discovered by Kepler about the 
year 1609 ; and hence are called Kepler's Laws. They 
are, 

1. The planets' orbits are ellipses having the sun or 
central body in one of the foci. 

2. The radius-vector of a planet's orbit passes over 
equal areas in equal times. 

71. What is meant by Kepler's Laws ? Recite them. What is meant "by the 
periodic time of a planet ? 




60 



ORBITAL REVOLUTIONS 



3. The squares of the periodie times of the planets are 
in proportion to the cubes of their mean distances from 
the sun or central body. 

By the periodic time of a planet is meant the time which it takes 
to revolve around the sun. The mean distance is the average dis- 
tance, found by adding together its greatest and least distances 
aud dividing by two. 

72. By the radius-vector of a planet's orbit is meant a 
line which joins the planet at any point of its orbit to 
the body around which it is revolving. The point of a 
planet's orbit nearest the sun is called its 'perihelion ; 
the point farthest from the sun is called its aphelion* 
These two points are sometimes called the apsides. 

o If S represent the sun in the 

locus of a planet's ellijDtical orbit, 
A will be the aphelion, P the 
perihelion, and A S, B S, C S, etc., 
the radius-vector in different po- 
sitions of the planet. The planet 
moves in its orbit so that the 
spaces A S B, B S C, etc., may be 
equal, if described in equal times. 
It has therefore to move much 
faster in the perihelion than in the 

aphelion, since at the former point the spaces must be wider in 

order to make up for their diminished length. 

73. The velocity of a planet is variable, baing greatest 
at the perihelion, least at the aphelion, rnd alternately 
increasing and diminishing between these two points. 




ELLIPTICAL, ORBIT. 



72. What is the radius-vector of a planet's orbit? What is perihelion? Aphe- 
lion ? What are the apsides ? 73. How does the velocity of a planet in its orbit 
vary ? When would it be uniform ? Why ? In the case of what bodies is this true ? 



OF THE PLANETS. 



61 



If the planets moved in circular orbits, the velocity 
would be uniform, since the radius-vector being con- 
stantly the same, the arcs, as well as the areas, described 
in equal times would be equal. This is the fact in the 
case of the satellites of Jupiter and Uranus, their orbits 
being almost, if not exactly, circles. 

74. The eccentricity of the large planets' orbits is very 
small, — that of Mercury, which is the largest, being only 
about J, while that of Yenus, the smallest, is only about 
tjg. The orbits of the minor planets are generally very 
remarkable for their great eccentricity. The eccen- 
tricity of an orbit is measured by comparing it with one- 
half of the major axis. 

Thus, when it is said that the eccentricity of mercury's orbit is 
i-, it is meant that it is ^ of the major axis. 

The annexed diagram will aid in 
giving the student a correct idea of 
the figure of the planets' orbits. 
This diagram represents an ellipse, 
the eccentricity of which is -*-, or 
much greater than that of the most 
eccentric of the minor planets. It 
will be apparent, therefore, that the 
actual fijurs of the planets' orbits is 
but slightly different from that of a 
circle. If drawn on paper, the eye 
could not detect the difference. 

75. The mean jplace of a planet is that in which it 
would be if it moved in a circle, and, of course, with 




ELLIPSE— ECCENTRICITY. \ 



74. How great is the eccentricity of the planets 1 orbits ? How is eccentricity 
measure! ? 75. What is the mean place of a planet ? The true place ? The equa- 
tion of the centre ? Illustrate by the diagram. 



62 ORBITAL REVOLUTIONS 

uniform velocity ; the true place is that in which it is 
actually situated at any particular time. The angular 
distance of the true place from the mean place, measured 
from the sun as a centre, is called the Equation of the 
Centre, 




MEAN AND TRUE PLACES OF A PLANET. 

In the above diagram, the ellipse represents the actual orbit of 
the planet, and the dotted circle the corresponding circular orbit. 
The points marked T represent the true places, and those marked 
M, the mean places of the planet. The mean place is obviously 
before or east of the true place, as the body moves from aphelion 
to perihelion, and behind, or west of it, in the other half of its rev- 
olution. The equation of the centre is the angle contained between 
the radius-vector and the radius of the circle. 

76. The planets do not revolve around the sun all in 
the same plane, but in planes slightly inclined to each 
other. The angle which the plane of a planet's orbit 

76. Do the planets revolve in the same plane ? What is meant by inclination of 
orbit? Which planet has the greatest? Which the least? Explain by the 
diagram. 



OF THE PLAXETS. 



63 



makes m ith that of the earth's orbit is called the Incli- 
nation of its Orbit. Of all the primary planets, Mercury 
has the greatest inclination of orbit (7°), and Uranus the 
least (i6 f ). The Minor Planets are remarkable for a 
much greater inclination of their orbits than that of 
the other planets. 




rSCLTNATIOX OF PLAXETS ORBITS. 



The diagram represents the j)osition of the plane of each orbit 
in relation to that of the earth. The small amount of deviation 
from one uniform plane wall be at once apparent. These planets, 
however, on account of their vast distance from the sun, depart 
very far from the plane of the earth's orbit. Thus, Mars, although 
having only 2° of inclination, may be nearly 5 millions of miles 
from this plane ; and Neptune, about 85 millions. 

77. Since the planets' orbits are all inclined to that of 
the earth, each of them must cross the plane of the 
ecliptic in two points. These two points are called the 
Nodes. The point at which the planet crosses from 
south to north is called the Ascend in g Node ; that at 
which it crosses from north to south, the Descending 



77. What are nodes? Which is the ascending node? The descending node? 
What is the line of nodes ? 



64 'DISTANCES OF THE PLANETS 

Node. The straight line which joins them is called 

the L'me of Nodes. 




INCLINATION OF ORBITS. 



The diagram represents an oblique view of the orbits of the earth 
and Venus. E is the ascending, and F the descending node. E F 
the line of nodes, and A S G ihe angle of inclination of the orbit. 
Q is the sign of the ascending node ; "Q , of the descending node. 



DISTANCES OF THE PLANETS FROM THE SUN. 

78. The planets are all at immense distances from the 
sun, that of the nearest planet being more than 85 mil- 
lions of miles. 

A million is so vast a number that we can form no true concep- 
tion of it without dividing it into portions. To count a million, at 
the rate of 5 per second, would require about 2J- days, counting 
without intermission, night and day. A railroad car, traveling at 
the rate of 30 miles per hour, night and day, would require nearly 
four years to pass over a million of miles. 

79. The following are the mean distances expressed i:i 
approximate round numbers : 



78. How far are the planets from the sun? 
planet ? Hlustratlon ? Bodes Law ? 



70. What is the distance of each 



FB03I THE SUN. 65 

Mercury, . 35 millions. Jupiter, . 476 millions. 
Venus, . 06 ' : Saturn, . 872 " 

Earth, . 91 J " Uranus, . 1,754 " 

Mars, . 139 " Neptune, 2,716 " 

Minor Planets, . . 260 millions (average). 

Illustration. — Multiply each of these numbers expressing mil- 
lions by four, and we shall find the time which an express train 
starting from the sun would require to reach each of the planets. 
In the case of the nearest planet, this period would be 140 years, 
and of the most remote, almost 11,000 years. A cannon ball mov- 
ing at the rate of 500 miles an hour would not reach Neptune in 
less than 626 years. 

Bode's Law. — A comparison of the distances given above will 
show a very curious numerical relation existing among them, each 
distance being nearly double that next inferior to it. A more ex- 
act statement of this numerical relation was published in 1772 by 
Professor Bode, of Eerlin, although not discovered by him ; it has 
usually been designated " Bode's Law." Take the numbers 

0, 3, 6, 12, 24, 48, 96, 192, 384; 
each of which, excepting the second, is double the next preceding ■ 
add to each 4, and we obtain 

4, 7, 10, 16, 28, 52, 100, 196, 388; 
which numbers very nearly represent the relative proportion of the 
planets' distances, including the average distance of the Minor 
Planets. In the case of Neptune, the law very decidedly fails, and, 
consequently, has ceased to have the importance attributed to it 
before the discovery of this planet in 1846. 

PERIODIC TIMES OF THE PLANETS. 

80. The following are the periods of time occupied by 
the planets respectively in completing one revolution 
around the sun : 



. What is the periodic time of each of the planets ? 

9 



66 PERIODIC TIMES OF THE PLANETS. 

Mercury, . SS days. Jupiter, . 12 yrs. (nearly). 

Yenus/ . 224J " Saturn, . 29 J " 

Earth, . . 305J " Uranus, . 84 t; 

Mars, . . 1 yr. 322 days. Neptune, 165 " 

81. Of all the primary planets, Mercury moves in its 
orbit with the greatest velocity, and Neptune with the 
least, the velocities of the planets diminishing as their 
distances from the sun increase. 

This is in accordance with Kepler's third law ; since the ratio of 
the periodic times increases faster than that of the distances, the 
square of the former being equal to the cube of the latter. Thus 
if the distance of one planet is four times as great as that of 
another, the periodic time will not be simply four times as long, 
but eight times as long ; that is, the square root of the cubd 
(V5 3 — 764 — g). Hence, as the planet has a longer time in pro- 
portion to the distance traveled, its velocity must be diminished. 

82. The following > exhibits the mean hourly motion 
of the primary planets in their orbits : 

Mercury, . . 104,000 miles. Jupiter, . . 29,000 miiea: 

Yenus, . . . 77,000 u Saturn, . . 21,000 " 

Earth, . . . 65,500 " Uranus, . . 15,000 " 

Mars, .... 53,000 " Neptune, . 12,000 " 

Illustration. — What an amazing subject for contemplation does 
this table present ! For example, the weight of the earth in tons 
is computed to be about 6,000,000,000,000,000,000,000 ; that is to 
say, six thousand million million times a million, or G,000 x 
1.000,000 x 1,000,000 x 1,000,000. Yet this body, so inconceivably 
vast, is rushing through the abyss of space with a velocity of 1,000 



81. Which planet moves with the greatest velocity ? Which with the least ? 
Illustration ? 82. State the mean hourly motion of e.-.ch planet in its orbit. Illus- 
tration ? How to find the hourly motion ? 



AXIAL ROTATIONS OF THE PLANETS. 67 

miles J3er minute, or about 15 miles during every pulsation of the 
heart. But the earth in comparison with the body around which 
it is revolving is as a single grain of wheat compared with four 
bushels. 

To Find tho Hourly Motion. — This can be done by the aj^pli- 
cation of very simple principles. The orbits being nearly circles, 
twice the mean distance will give us the diameter, and 3f times 
the diameter will give the circumference, or whole distance trav- 
eled in the periodic time. Then finding the number of hours in 
this time, and dividing the whole distance by this number, we 
obtain the hourly motion. Thus, Mercury's mean distance is 35 
million miles ; then 35 x 2 x 3^- = 220 millions, the whole dis- 
tance traveled in 88 days, or 88 x 24 = 2112 hours; and 220 mil- 
lions -^- 2112 = 104,166 miles. 

AXIAL ROTATIONS OF THE PLANETS. 

83. Besides revolving around the sun, the planets re 
volve upon their axes in the same direction as they 
revolve in their orbits ; that is, from west to east. 
This is called their diurnal rotation. Proofs of the 
earth's rotation have already been given; that of the 
other planets is indicated by a regular movement of 
spots across their disks. 

Let the pupil stand at a distance from a globe, and let it be re- 
volved, and he will observe the marks upon it move across, and 
alternately disappear and reappear. The same thing must, of 
course, occur in our observation of the planets, if they have a 
diurnal motion. 

84. The times of rotation of the planets respectively 
are as follows : 

83. What is meant by diurnal rotation ? How is the rotation of a planet indi- 
cated? Illustration? 84. What are the times of rotation of the planets, respec- 
tively ? 



68 AXIAL ROTATIONS OF THE PLANETS. 

Mercury, . . 24J hours. Jupiter, . . 10 hours. 

Venus, . .231 - Saturn, . . 10 J " 

Earth, . . 21 " Uranus, . . 9£ " (?) 

Mars, . . 24J " Neptune, . (unknown.) 

It will be observed that the terrestrial planets all perform their 
rotations in about 24 hours ; but that the major planets require 
less than one-half that time. 

85. Every planet must rotate with its axis either per- 
pendicular or oblique to its orbit. The axes of the 
planets are all considerably oblique, excepting that oi 
Jupiter, which is only 3° from the perpendicular; that 
of Yen us is supposed to be very oblique. The angle 
which the axis of a planet makes with a perpendicular 
to its orbit is called the Inclination of Axis. 

86. The inclination of the axis of each planet, as far 
as it has been discovered, is as follows : 

. Unknown. Jupiter, . 

. 75° (?) Saturn, . 

. 23|°. Uranus, . 

. 281°. Neptune, 



Mercury, 
Yenus, 
Earth, 
Mars, 



3°. 
26f°. 
Unknown. 






INCLINATION OF JTPITER. EARTH, AND TENTS. 



85. What is the position of the planets' axes ? What is meant by inclination of 
axis ? 86. State the inclination of each. 



ASPECTS OF THE PLANETS. 



69 



87. Sun's Rotation. — The sun also rotates upon an 
axis, but requires about 60S hours, or 25 J days, to com- 
plete one rotation. The inclination of its axis to the 
plane of the ecliptic is 7J°. 



ASPECTS OF THE PLAXETS. 



88. The aspects of the planets are their apparent 
positions with respect to the sun and each other. The 
aspects most frequently 



referred to are conjunc- 
tion, quadrature, and 
opposition. 

89. A planet is said 
to be in conjunction 
with the sun when it is 
in the same part of the 
heavens. That is, if the 
sun is in the east, the 
planet must also be in 
the east, both being pre- 
cisely in the same direc- 
tion. 



ft£~?£S*>+ 






SUPERIOR 



INFERIOR 



Opposition 
aspects. 






90. Conjunction may be inferior or superior. In- 
ferior conjunction is that in which the planet is between 
the earth and the sun ; superior conjunction is that in 



87. In what time does the sun rotate ? What is the position of its axis ? 88. What 
are aspects ? Which are the most frequently referred to ? 89. When is a planet in 
conjunction with the sun ? 90. Of how many kinds is conjunction ? What is inferior 
conjunction ? Superior conjunction ? 



70 



ASPECTS OF THE PLANETS. 



which the planet is on the opposite side of the sun from 
the earth. 

91. A planet is said to be in opposition when it is in 
the opposite part of the heavens from the sun. That is, 
while the sun is in the east, the planet must be in the 
west, It is obvious that only the superior planets can be 
in opposition. 

92. These aspects depend upon the angular distance 
of the planets from the sun (Introduction, Art, 17). In 
conjunction, there is no angular distance, except what 
is caused by the inclination of the orbits ; in opposition, 
the angular distance amounts to 180°. The angular 
distance of a planet from the sun is called its Elongation. 

93. When the elongation of a planet is 90°, it is said 
to be in quadrature. This position in the heavens is 
half-way between conjunction and opposition ; but in 




91. When is a planet said to be in opposition ? 92. What is the angular distance 
in conjunction and opposition? What is elongation? 93. When is a planet in 
quadrature ? Illustrate from the diagram. 



APPARENT MOTIONS OF THE PLANETS. 71 

the planet's orbit it is much nearer opposition than con- 
junction (see diagram, page G9). 

In the diagram, the graduated semicircle cuts the sides of all the 
angles which have their vertices at E, and serves to measure the 
angular distance of each planet from the sun. V and V" represent 
Venus in superior and inferior conjunction, the elongation being, at 
those points, 0° ; while at V, it is at its point of greatest elonga- 
tion. It will be obvious from this diagram that no inferior j;>lanet 
can be 90° from the sun. M represents Mars in opposition, and M' 
the same planet in quadrature. The aspect of M and V or V" is 
opposition ; of M' and V or V', quartile. 

94. The time that elapses between two similar elonga- 
tions of a planet is called its Synodic Period. 

The term is generally applied to the interval between two suc- 
cessive inferior or superior conjunctions. The synodic period would 
be the true period of a planet's revolution around the sun, if the 
earth were at rest ; but the earth is moving in its orbit in the same 
direction as the planet, with a velocity less than that of the inferior 
planets, and greater than that of the superior planets. Hence, the 
synodic period of an inferior planet is greater than the periodic 
time, since, after completing a revolution, it has to overtake the 
earth in order to reach the same relative position with the sun. 
The synodic period of a superior planet is generally less than its 
periodic time, since the earth, after performing one revolution, 
overtakes the planet before its revolution is completed. This is 
true of every superior planet except Mars. 

APPARENT MOTIONS OF THE PLANETS. 

95. The planets generally appear to move among the 
stars from west to east ; but sometimes the apparent 
motion is from east to west. In the former case, the 
motion is said to be direct ; in the latter, retrograde. 

94. What is the synodic period of a planet ? Why is the synodic period of an in- 
ferior planet greater than the true period ? Why is that of a superior planet less ? 
95. What apparent motions have the planets ? When is the motion direct ? When 
retrograde ? When is a planet said to be stationary ? 



72 APPARENT MOTIONS OF THE PLANETS. 

At certain intermediate points of its course, the planet 
remains for a short time in the same point of the 
heavens, and is then said to be stationary. 

96. An inferior planet appears stationary at two points 
of its synodic revolution, between its extreme elonga- 
tions and inferior conjunction. This is caused by the 
direction of its motion at these two points, it being 
oblique to the motion of the earth ; so that, notwith- 
standing the difference in their velocities, the planet and 
earth appear to move on together. 

97. The motion of an inferior planet is retrograde 
while it is passing through inferior conjunction from one 
stationary point to the other, and direct in passing through 
superior conjunction between the same two points. 




APPARENT MOTIONS OF VENUS AND MERCURY. 



96. At what points is a planet stationary ? How is this caused? 97. When is 
the motion of an inferior planet retrograde ? Illustrate from the diagram. 



A P P ARE X T 3I0TI0XS B\ THE P LA XE TS. 73 

Let S in the annexed diagram be the place of the sun, the inner 
circle the orbit of an inferior planet ; the outer circle that of the 
earth. Let also a, o, e, fZ, etc., be the positions of the planet at 
unequal intervals of time between the points of extreme elongation, 
a and g ; and A, B, C, D, etc., the places of the earth at the same 
time ; while 1, 2, 3, 4, etc., represent the apparent places of the 
planet, as seen in the sphere of the heavens. In passing from g, 
the western point of extreme elongation, through o, the place of 
superior conjunction, to a, the eastern point of extreme elongation, 
the planet evidently niUGt appear to move toward the east; and 
when it arrives at a, the earth being at A, it still continues to be 
direct for a short time ; for while going from a to o its motion is so 
oblique that the earth, passes it, so that when the latter arrives at 
B, the planet appears to have moved from 1 to 2. Its elongation 
is not, however, increased since the sun itself has moved farther to 
the east. While the planet is going from h to c, and the earth from 
B to C, the former does not appear to change its position at all ; 
for the lines B 5 2 and C c 3 are parallel, and consequently indi- 
cate no change of place among the stars, and 2 is to be considered, 
therefore, as identical with 3. The reason of the planet's appear- 
ing stationary, it will be seen, is that the obliquity of its motion 
exactly counterbalances the difference between its actual velocity 
and that of the earth ; b is, therefore, to be considered the station- 
ary point. At d, the planet is in inferior conjunction, having over- 
taken the earth, and is seen at 4, to the west of its previous posi- 
tion. In passing from d to g the same phenomena are presented m 
the reverse order ; at e it becomes stationary, remaining so till it 
reaches /, where it ceases to be retrograde, appearing to move,, 
while going from/ to g, from 6 to 7. In going from c to e, the two 
stationary points, it has evidently changed its direction among the 
stars, not by the actual distance 3, 5, but by the angle contained by 
the lines 3 c C and 5 e E when produced until they meet in some 
point below CDE. This angle, or the arc by which it is subtended, 
it is obvious, is quite small ; it is called the arc of retrogradation. 

98. The motion of a superior planet appears to be 
retrograde for a short distance before and after opposi- 
10 



7Jf QUESTIONS FOR EXERCISE. 

tion, and direct in the other part of its orbit. The retro- 
gradation of the planet is caused by the greater velocity 
of the earth ; so that as the latter body moves toward 
the east, it passes the other, and thus makes it appear to 
move toward the west. When the motion of the earth 
is sufficiently oblique to counteract the excess of its 
velocity, the two bodies move on together, and the planet 
appears to be stationary. 

QUESTIONS FOR EXERCISE. 

1. When a planet is in quadrature, what is its elongation ? 

2. What is its elongation when in inferior conjunction ? 

3. What is its elongation in superior conjunction ? 

4. How many degrees of elongation has it when in opposition ? 

5. Which of the planets can be in inferior conjunction ? 

6. Which can be in superior conjunction ? 

7. Which can be in opposition ? 

8. Which can be in quadrature ? 

9. Can the elongation of Mercury or Yenus exceed 90° ? 

10. Can that of Jupiter ? 

11. What is the greatest elongation of a superior planet ? 

12. When Venus is in inferior conjunction, and Mars in opposi- 
tion, what is their angular distance from each other ? [See Fig., 
p. 70.] 

13. What is their angular distance when Venus is in inferior con- 
junction, and Mars in superior conjunction ? 

14. How many degrees are they apart when Venus is in superior 
conjunction and Mars is in quadrature ? 

15. When the elongation of Venus is 30°, and that of Mars is 
120°, what is their angular distance from each other ? 

16. If Venus is 50° from Mars, and the latter body is in quadra- 
ture, what is the elongation of Venus ? 

98. When is the motion of a superior planet retrograde ? How caused ? 



SECTION IV. 

DESCRIPTION OF THE SUN AND PLANETS. 

99. The Sun is the source of light and heat to all the 
planets, and, in connection with the atmosphere, the 
support of life and vegetation on the surface of the earth. 
The forces displayed on our planet, which spring from 
its exhaustless rajs, are inconceivably great ; and jet, it 
is calculated that the earth, with its limited grasp, only 
receives the two hundred millionth part of the whole 
force radiated and dispensed by this vast and splendid 
luminary. 

100. The apparent diameter of the sun, or the angle 
which it subtends in the celestial sphere, is a little more 
than one-half of a degree. This is, of course, greater 
when the earth is in perihelion than when it is in aphe- 
lion, the variation in the apparent size of the sun afford- 
ing a means of ascertaining the figure of the earth's orbit. 
The real diameter of the sun is S52,900 miles. 

The greatest apparent diameter of the sun is 32'.6, and the least 
Sl'.533 ; hence, the ratio is 1.034 : 1 (nearly), and one-half the differ- 
ence, or .017 is about the eccentricity of the orbit. 

101. The apparent diameter of the sun at each of the 
planets diminishes in proportion as the distance increases. 

99. What is said of the sun ? Of the forces derived from its rays ? 100. How 
great is the apparent diameter of the sun ? How does it vary ? 101. How does it 
differ as seen from the other planets ? How great is the sun's light at the earth ? 



76 



DESCRIPTION OF THE 



Thus, at Mercury it is 2J times as great as at the earth ; 
but at Xeptune it is only ^ as great. Various experi- 



£ARTH 
||^ MARS 



• * 



APPAKENT MAGNITUDES OF THE SUN. 

ments seem to show that the light of the sun at the earth 
is equal to that of 600,000 full moons. 

102. The distance of the sun from the earth when the 
latter is in aphelion is about 93 millions of miles ; but 
w T hen it is in perihelion the distance is only 90 millions ; 
the mean distance, as previously stated, being about 91 £ 
millions. 

The distance of the sun from the earth has been, from the earliest 
times, a subject of careful and earnest investigation with astrono- 
mers. Ptolemy and his contemporaries supjDosed it to be only 
1200 times the radius of the earth, or less than five millions of 
miles. It was not until the middle of the last century that this 
was discovered with any degree of accuracy. The above is quite a 
recent determination. 

103. Solar Spots.— When the disk of the sun is ex- 
amined with a telescope, it is found to exhibit certain 

102. What is the distance of the sun from the earth ? Remark ? 103. What does 
the sun's disk exhibit when viewed with a telescope ? What is said of the position 
and appearance of the solar spots? What is the black portion called ? The dusky 
border? 



SUN AND PLANETS. 77 

dark spots, which constantly vary in number, form, size, 
and general appearance. These spots are sometimes very 
numerous, beins; mostly confined to the region of the 



ITS- : 



SOLAK SPOTS GREATLY 3IAGXXFLED. 

sun's equator, and some of them are of vast size. They 
look like irregular black patches surrounded with a 
dusky border or fringe. The black portion in the centre 
is called the umbra or nucleus ; the dusky border, the 
penumbra. 

104. Sometimes the sun's disk is entirely free from 
them, and continues so for weeks or even months ; 
at other times, they seem to burst forth and spread 
over a certain part of it in great numbers. The dura- 
tion of single spots is also exceedingly variable. A spot 
has been seen to make its appearance and vanish entirely 
within twenty-four hours ; while others have continued 
for nine or ten weeks without much change of appear- 
ance. 

104. What change? occur in their number and appearance ? What is said of their 
duration ? 



78 



DESCRIPTION OF THE 



105. Various hypotheses have been advanced to ac- 
count for these spots. The most recent is, that the sun 
is a solid or liquid mass in an intensely heated condition, 
surrounded by an atmosphere of vapor or burning gas ; 
and that the disturbances in this atmosphere cause the 
spots, which are in fact only openings in it filled with 
clouds of various degrees of density. 



- — r - 



■i -ll 



A SPOT PASSING ACROSS THE DISK. 



106. There is a general movement of the spots across 
the sun's disk from east to west. A particular spot first 
appears on the eastern limb or edge of the disk, passes 
across to the western limb, and then disappears, but 
after about two weeks reappears on the eastern limb, 
completing an entire revolution in about 2TJ days. 




MOVEMENT OF THE SUN AND SPOTS. 



105. What hypothesis is advanced to account for them ? 106. How do the spots 
appear to move ? What is the time of a complete revolution ? What is inferred 
from this ? 



£ UN A XD PL A NE TS. 



79 



From tliis we infer that the sun rotates on its axis in 
25^ days. 

107. A revolution of the spot must take longer than 
one rotation of the sun in consequence of the earth's 
motion in its orbit in the same direction. 

[The diagram will make this intelligible. Let A be the position 
of the earth in its orbit at the time a spot appears at C. Now, 
while the sun performs a rotation, the earth moves on to B ; and 
hence the spot must pass from C to D to become visible.] 



THE ZODIACAL LIGHT. 



108. Apparently connected with the sun is the singu- 
lar phenomenon called the 
Zodiacal Light. This is a 
flint luminous appearance, 
of the form of a triangle or 
cone, seen at certain seasons 
of the year, in the evening 
at the western horizon, and 
in the morning at the east- 
ern. It extends obliquely 
from the horizon in the plane 
of the sun's equator, its 
breadth at the horizon vary- 
ing from eight to thirty de- 
grees. 




107. Why is the time of the spots' revolution greater than that of the sun's rota- 
tion ? Explain from the diagram. 108. What is the Zodiacal Light ? How does it 
extend ? What is its position ? Its breadth ? 



80 THE ZODIACAL LIGHT. 

109. It is seen most distinctly in March and April, 
after sunset, and in September and October, before sun- 
rise ; because at those times the ecliptic is most nearly 
perpendicular to the horizon. 

110. Various hypotheses have been suggested to ac- 
count for it. That most generally received is, that it 
is a nebulous mass of the shape of a lens, encompass- 
ing the sun at its equator, and extending sometimes 
beyond the orbit of the earth. Some have regarded it 
as a vast ring of meteors circulating about the sun, 
some of which are constantly falling into it. 

This hypothesis of a constant shower of meteoric bodies falling 
upon the sun has been used to account for the support of its heat ; 
for their collision with the sun would necessarily generate an in- 
tense heat, just as iron may be heated to any degree by hammering 
it. It is calculated that bodies of the density cf granite falling all 
over the sun to the depth of 12 feet in a year, and with the velocity 
which they would acquire (384 miles in a second), would maintain 
the solar heat. If Mercury were to strike the sun, it would gen- 
erate an amount of heat equal to all the sun emits in seven years ; 
while the shock of Jupiter would supply the loss of more than 
30,000 years. 

MEKCUCT « . 

111. Mercury is remarkable for its small size, its 
swift motion, and the great inclination and eccentricity 
of its orbit. It is, as far as is positively known, the 
nearest planet to the sun. 



109. Where is it seen most distinctly ? Why ? 110. What hypotheses are ad- 
vanced to account for it ? Theory to account for the eun'B heat? ni. For what ia 
Mercury remarkahle ? Is there a planet nearer to the sun ? 



MERCURY. 81 

A planet inferior to Mercury has been supposed to exist ; and in 
1859, a French astronomer was thought by some to have discovered 
it. Later observations have not. however, confirmed, but rather 
disproved, its existence. The name given to this supposed planet 
is Vulcan. 

112. Mercury can only be seen in the morning or 
evening, as it appears to keep close to the sun, never de- 
parting from it more than 28°. It is very brilliant — so 
much so, indeed, that the ancients called it the " Twink- 
ler." It was called Mercury in consequence of the 
swiftness of its motion ; for in the ancient mythology 
of the Greeks, Mercury is the messenger of the gods. 
The sign g is supposed to represent the wand whicli 
Mercury carries in his hand. 

113. Orbit. — Mercury's orbit is very eccentric; so 
that, in aphelion, the planet is nearly 15 millions of 
miles farther from the sun than in perihelion. Its in- 
clination is about 7°, being greater than that of any of 
the other large primary planets. 

114. Volume, Density, &c. — The diameter of Mer- 
cury being a little less than 3,000 
miles, its volume is only about 
one-twentieth that o'f the earth ; 
but its density is somewhat 
greater. A body that weighs a 

, in r> .1 COMPARATIVE VOLUMES OP 

pound on the surlace ol the ME rcury and the earth. 



112. When can Mercury be seen ? How far does it appear to depart from the sun ? 
Why was it called the ''Twinkler? " Origin of its name ? Its sign ? 113. What is 
said of the eccentricity and inclination of its orbit ? 114. What is its volume as 
compared with the earth ? What difference in the weight of a body at the surface 
of Mercury? 

11 




earth would weigh less than one-half a pound at Mer- 
cury ; and consequently, if we were transported to that 
planet, our muscular power would practically be doubled. 

115. Telescopic Appearance. — When viewed through 
a telescope, this planet exhibits all the phases of the 
moon. Indications of the existence of very high moun- 
tains, and of an atmosphere of considerable extent, have 
been detected by some observers ; but the excessive bril- 
liancy of the planet makes it an exceedingly difficult 
object for telescopic observation. Sir John Herschel 
states that all that can be certainly affirmed of it is, that 
"it is globular in form and exhibits phases; and that it 
is too small and too much lost in the constant and close 
effulgence of the sun to allow the further discovery of its 
physical condition." 

VENUS Q . 

116. Venus is the most brilliant and beautiful of all 
the planets, and is remarkable for its resemblance to the 
earth both in size and mass. By the ancients, it was 
called Hesperus or Vesper when an evening star, and 
Phosphorus or Lucifer when a morning star, these being 
at first supposed to be different bodies. It derives its 
name from Yenus, the goddess of beauty ; and the sign 
represents a mirror, with the handle at the lower side. 

117. Orbit — The eccentricity of its orbit is quite 
small, the difference between its aphelion and perihelion 
distance being less than one million of miles. Its meli- 



us. What is said of the telescopic appearance of Mercury ? 116. What is said of 
Venus ? Its name and sign ? 117. The eccentricity and inclination of its orbit ! 



VENUS. 88 

nation of orbit is only about one-half that of Mercury, 
being less than 3 J degrees. 

118. Sidereal and Synodic Period.— Since the side- 
real or true period of this planet's revolution around the 
sun is nearly 225 days, its synodic period is about 585 
days ; because, on leaving the earth after inferior con- 
junction, it has to perform more than two revolutions 
before it overtakes it so as to be again in the same rela- 
tive position with the earth and sun. 

This will be evident if we consider that while the earth is com- 
pleting one revolution, Venus performs one and five-eighths of 
another. But it has to gain an entire revolution to overtake the 
earth ; and if it takes it 365|- days to gain -f, it will take it 584|- 
days to gain the whole. 

119. Telescopic Appearance. — The telescopic ap- 
pearance of Venus is very beautiful, owing to its phases, 
which exactly resemble those of the moon. When it is 




TELESCOPIC VIEWS OF VENUS. 

in superior conjunction, its full disk is visible, which 
gradually diminishes until, at the time of greatest elon- 

118. What is its sidereal period ? Its synodical period ? Why is the latter longer ? 
How calculated ? 119. What is said of its telescopic appearance ? Its phases ? Its 
apparent diameter ? How are irregularities on its surface indicated ? Its atmos- 
phere ? 



gation, only half of the disk can be seen. After this 
the planet still continues to wane, until, when near in- 
ferior conjunction, it assumes the form of a slender 
crescent. Its apparent diameter when in inferior con- 
junction is more than six times as great as it is when in 
superior conjunction. 

Yery great irregularities on its surface are indicated 
by the jagged character of the terminator, or line bound- 
ing the illuminated part of the disk, as w T ell as by the 
shading of its edge, caused by the shadows of the moun- 
tains as the sun's rays fall obliquely upon them. Like 
Mercury, this planet is a difficult object for observation, 
owing to its excessive brilliancy. There are undoubted 
indications that it possesses an atmosphere of consid- 
erable height and density. 

120. Seasons. — The seasons of Venus are quite re- 
markable, owing to the great inclination of its axis. 
This being 75°, its tropics must be 75° from its equator, 
and its polar circles 75° from its poles. Hence it can 
have only a torrid zone, which must be 150° wide, and 
frigid zones extending 75° from the poles. There must 
be, therefore, two summers and two winters at the equa- 
tor, and a summer and winter alternately at each pole 
during the year, the latter lasting about 112 days. 

The annexed diagram exhibits the planet at each of the equi- 
noxes and solstices. To an inhabitant of* the northern hemisj^here 
of Venus, at A the sun is in the vernal equinox ; B, the summer 
solstice ; C, the autumnal equinox ; and D, the winter solstice. 



120. What seasons at Venus? How are its tropics and polar circles situated? 
Illustrate from the diagram. 





. 






MA R S . 








85 


When the sun is in 


the nortl 


iern solstice, 


it will be 


seen 


that all 


places situated 


more 


! than 15 c 


north of the equator 


have constant 


Say, and those 


more 


than 15 


south of it, 


constant 


night. 


Hence 


H 


HQjgggjgH 
















jm 


W\ 


ESG9 




E^^^^^w 


^S 


\ 


osra 










TSFfjT 









SEASONS OF YENTJS. 



there must be winter at the equator and within the south polar 
circle, and summer within the north polar circle. In one-fourth of 
the year, when the sun will have arrived at the equator, there will 
be equal day and night all over the planet, summer at the equator, 
autumn within the north polar circle, and spring within the south 
polar circle. 

MAES $ . 

121. Mars, the fourth, planet from the sun, is remark- 
able for its small size and the red color with which it 
shines among the stars. It is consequently very easily 
distinguished from the other heavenly bodies, and doubt- 
less received its name on account of this red appearance, 
Mars being the god of war. Its sign is a shield and 
spear. 



121. For what is Mars remarkable ? Origin of its name ? Its s 



86 2IARS. 

122. Orbit. — The eccentricity of its orbit is nearly 5^ 
times as great as that of the earth's, it being about 20 
millions of miles nearer to the sun in perihelion than in 
aphelion. Its inclination of orbit is about a degree and 
a half. 

Owing to the great eccentricity of the orbit of Mars, it some- 
times, when in opposition, approaches very near to the earth ; for 
if it is in perihelion while the earth is in aphelion, the distance is 
126,300,000 — 93,000,000 = 33,300,000. 

123. Seasons. — Since the inclination of its axis is 
nearly the same as that of the earth, the variety of sea- 
sons must be the same ; but they must be nearly twice 
as long, because it takes Mars nearly two years to re- 
volve around the sun. Owing to the great eccentricity 
of its orbit, summer in the northern hemisphere must be 
only four-fifths as long as in the southern ; and at each 
of the poles there must be, alternately, constant day and 
constant night, each lasting nearly one of our years. 

124. Telescopic Appearance. — The telescopic ap- 
pearances of this planet are very interesting, exhibiting 
w T hat seem to be the outlines of continents and seas, the 
former appearing of a ruddy or orange color, and the 
latter of a dusky greenish or bluish tint. Brilliant white 
spots are also seen alternately at the poles, evidently pro- 
duced by accumulations of ice and snow during the long 
winters, particularly as they are seen to disappear as 

122. How great is the eccentricity of its orbit ? Its inclination ? 123. What son- 
sons has Mars ? Why ? Why is summer shorter in the northern than in the south- 
ern hemisphere ? Duration of constant day and night at the poles ? 124. What does 
its telescopic appearance exhibit ? Where are brilliant white spots ? How caused ? 
Cause of the red color of Mars ? Appearance of continents and islands in the cut ? 



87 



summer advances. Evidences are also presented of an 
atmosphere, probably equal in density to that of the earth. 

No entirely satisfactory cause has been assigned for the ruddy 
color of this planet. It is thought by Sir John Herschel to be due 
to " an ochrey tinge in the general soil, like what the red sandstone 
districts on the earth may possibly offer to the inhabitants of Mars, 
only more decided." Viewed through a telescope, the redness of 
its hue is very considerably diminished. 




NORTHERN AND SOUTHERN HEMISPHERES OF WARS, 



The annexed cut does not represent any actual telescopic views 
of the planet, since we are never so situated as to be able to see the 
whole of either the northern or southern hemisphere at any one 
time. It exhibits a combination of a large number of telescopic 
appearances, the various dusky spots being placed together so as 
to show the forms of the different bodies of water and their relation 
to the continents ; the latter being indicated by the white spaces. 
These, through the telescope, appear of a ruddy color, and give 
this general tint to the planet. On the earth, the continents arc 
islands, being encompassed by the water ; on Mars, it will be per- 
ceived, the bodies of water arc lakes or seas, being entirely encom- 
passed by the land. 



J UP I TEE. 



JUPITEE 



125. Jupiter, the first of the major planets, is remark- 
able for its great size, as well as for the peculiar splendor 
with which it shines among the stars. It doubtless re- 
ceived its name on this account, Jupiter being, in the 
ancient mythology, the king of the gods. Its sign is 
supposed to be an altered Z, the first letter of Zt -us, the 
name of Jupiter among the Greeks. 

126. Orbit. — The eccentricity of its orbit is nearly 
three times that of the earth, but its inclination is very 
small, being a little more than one degree (1° 19'). 

127. Figure. — The oblateness of its figure is very 
great, its equatorial diameter being 5,000 miles longer 
than its polar diameter. This is caused by the rapid ro- 
tation on its axis, which being performed in a little less 
than ten hours, a point on the equator of this planet 
moves with a diurnal velocity of 28,000 miles an hour, 
or 27 times as fast as at the earth. 

128. Volume, Density, etc.— The volume of Jupiter 
is nearly 1250 times as great as the earth's ; but as its 
mass is discovered to be only 300 times as great, its den- 
sity can be only about \ that of the earth, or a little 
greater than that of water. The force of gravity at the 
surface of the planet is about 2^ times as great ; so that a 

125. For what is Jupiter remarkable ? Origin of its name ? Its sign ? 126. now- 
great is the eccentricity of its orbit ? Its inclination ? 127. How great is the 
oblateness of its figure ? How is this caused ? 128. What is said of its volume, 
mass, and density ? Force of gravity at its surface V Velocity in its orbit ? 



JVPITEP.. 




COMPARATIVE MAGNITUDES OF THE EAETH AND 
JCPITEE. 



body weighing one pound at the earth's surface would 

weigh '2^ pounds at 

Jupiter's. 

This body, so incon- 
ceivably vast, is flying in 
its orbit with the ve- 
locity of 28,700 miles an 
hour, or nearly 500 miles 
a minute — a speed sixty 
times as great as that of 
a cannon ball. How tre- 
mendous is the exhibi- 
tion of force here dis- 
played ! 

129. Telescopic Appearance. — When examined with 
a telescope the disk of Jupiter appears crossed by dusky 
streaks or belts, parallel to its equator, their general 
direction always remaining the same, although they con- 
stantly vary in number, breadth, and situation on the 
disk. Sometimes the disk is almost covered with them ; 
while at other times scarcely any are visible. 

These dusky bands or belts are supposed to be the 
body of the planet seen between the clouds that con- 
fently float in its atmosphere, and are thrown into zones 
or belts by the great velocity of its rotation. The cloudy 
zones are more luminous than the surface of the planet, 
because they more perfectly reflect the solar light. 

The belts are not equally conspicuous, there being two generally 
which are more distinctly observable than others, and more perma- 



129. Describe its telescopic appearance. What are the belts supposed to be ? 
Their situation and appearance ? 

12 



90 JUPITER. 

nent. These are situated, one on each side of the equator, and are 
separated by a clear space somewhat more luminous than the other 
parts of the disk. Toward the poles they are narrower and less 
dark ; and they imperceptibly fade away a short distance from the 
eastern and western edges of the disk — a phenomenon clue evi- 
dently, to the thickness of the atmosphere at those parts. Dark 
spots are also occasionally seen in connection with the belts. 




TELESCOPIC VIEWS OF JUPITER. 



In the cut are given two telescopic views of this planet ; the 
first, from a drawing by Sir John Herschel, as it appeared Septem- 
ber 23d, 1832; the second, by Madler, in 1834. The two dark 
spots shown in the latter were employed to determine the time of 
the planet's rotation. 

130. Satellites. — The four srtellites of Jupiter are 
among the most interesting bodies of the solar system. 
They were first seen by Galileo, in 1610, a short time 
after the invention of the telescope, and were perceived 
to be satellites by their apparent movements with respect 
to the planet, alternately approaching it, passing behind 

130. What is said of the satellites ? How designated ? 



JUPITEE. 01 

it, and receding from it ; sometimes also passing over its 
disk and casting their shadows upon it, 

These planets have been distinguished by particular names, but 
are more generally designated by the numerals I., II., III., IV., 
according to their order from Jupiter. 

131. The orbits of these bodies are almost circular, and 
very nearly in the plane of the planet's equator. They 
therefore make only a very small angle with the plane 
of its orbit (about 3°). 

132. Their distances from Jupiter are, respectively, 
26±,000 miles, 423,000 miles, 678,000 miles, and 1,188,000 
miles . 

Their diameters in approximate numbers are I., 2,300 
miles; II., 2,070 miles; III., 3,400 miles; IV., 2,900 
miles ; all, excepting the second, being larger than the 
moon. 

As seen from Jupiter these bodies present quite large disks ; the 
apparent diameter of I. being 36' ; of II., 19' ; of III., 18' ; and of 
IV., 9'. The first is therefore somewhat larger in appearance than 
that of the moon. The firmament of Jupiter must present a very 
beautiful diversity of phenomena. These various moons, all of 
which are occasionally above the horizon at one time, go through 
their phases within a few days ; the first within 42 hours. To an 
inhabitant of the first satellite, the apparent diameter of Jupiter 
must be 19° ; that is, about 36 times as great as the moon ; while 
t!ie amount of illuminated surface p resented by it must be nearly 
1300 times as great. 

133. The eclipses, occultations, and transits of the 

131. What is the figure of their orbits ? Their position ? 132. Their distance i 
from Jupiter ? Their diameters ? How do they appear at Jupiter ? 133. What 
phenomena are presented by them ? What is said of their eclipses ? How do the 
transits occur ? Explain the diagram. 



92 JUPITER. 

satellites present an endless series of interesting and use- 
ful phenomena ; and the situation of their orbits causes 
them to occur with very great frequency. 

During the transits the satellites appear like bright spots passing 
from east to west across the disk, preceded or followed by their 
shadows, which seem like small round dots as black as ink. 



If.., 



:0" 



ECLIPSES, OCCULTATIONS, AND TRANSITS OF JUPITER S SATELLITES. 

In the figure, to an observer at E, I. is represented as eclipsed ; 
II., as just passing into the shadow of the planet; III., just before 
a transit, the shadow preceding ; and IV., at the point of occupa- 
tion. At E', I. has just passed behind the disk ; II. is in occupa- 
tion ; III., a transit, both shadow and satellite being on the disk, the 
shadow preceding ; IV., just emerging from behind the planet ; at 
E", I. and II. are behind the disk, III. is in transit, but the shadow 
follows the satellite ; IV., just after an eclipse. 

134. Since the occurrence of these eclipses can be 
exactly predicted, they serve to mark points of absolute 

134. What do the eclipses mark ? How is this useful t 



SATUHN. £3 

time ; so that if the precise moment at which they will 
occur at any particular place has been computed, and the 
actual time of their occurrence at any other place is 
noted, a comparison of the two will give the difference 
of time, and, of course, the difference of longitude, be- 
tween the two places. 

Thus, if a mariner perceives, by the nautical almanac, that the 
eclipse of a satellite will occur at 9 o'clock P.M., Washington time, 
and he notices that the eclipse does not take place till 11 o'clock 
P.M., he can infer that his position is 2 hours, or 30°, east of Wash- 
ington. 

135. Velocity of Light. — In the calculation of these 
eclipse.-, a constant variation was, for several years, found 
to exist between the calculated and the observed time 
of the occurrence, with the additional fact that the 
eclipse was later as Jupiter receded from the earth, and 
earliar as it approached the earth, being about 16J 
minutes earlier in opposition than in conjunction. This 
is explained by supposing that light requires 16-J minutes 
to cross the orbit of the earth, and hence that it passes 
to us from the sun in 8j- minutes. Its velocity must 
therefore be about 184,000 miles a second. 

This theory was promulgated in 1675 by Hans Roemer, a Danish 
astronomer, and has been confirmed by other and more recent 
discoveries. 

SATURN *? . 

136. Saturn i3 the centre of a very large and peculiar 
system, being attended by eight satellites and encom- 

135. What important discovery did this lead to ? In what way ? By whom made V 
106. What is said of Saturn ? Its name and sign ? 



9Jj. ISA TUB N. 

passed by several rings. It shines generally with a dull 
yellowish light. 

Saturn, in the ancient mythology, was one of the older deities, 
and presided over time, the seasons, etc. He was represented as a 
very old man carrying a scythe in one hand. The sign of the planet 
is a rude representation of a scythe. 

137. Orbit. — The eccentricity of its orbit is a little 
greater than that of Jupiter, being about ^V of its major 
axis. It is therefore nearly 100 millions of miles nearer 
to the sun in perihelion than in aphelion. The inclina- 
tion of its orbit is about 2| degrees. 

138. Figure, eto. — The oblateness of this planet is, 
like that of Jupiter, very remarkable, its equatorial diam- 
eter being 7,800 miles longer than the polar diameter. 
Its axial rotation being performed in 10| hours, its equa- 
torial velocity is more than 22,000 miles an hour ; and, 
as its density is somewhat less than that of oak wood, its 
figure would necessarily be very much flattened by the 
action of the centrifugal force. 

The density of this planet is so small that, notwithstanding its 
immense size, a body at its surface would weigh only about £ more 
than at the surface of the earth. 

139. Seasons. — As the inclination of its axis is about 
27°, its seasons must be similar to those of the earth ; 
but, in consequence of the length of its year, are nearly 
thirty times as long as on the earth. 

140. Telescopic Appearance. — This planet, when 

137. Eccentricity and inclination of its orbit? 138. How great is the oblateness 
of its figure ? Why ? Weight at its surface ? 139. What seasons has it ? 140. De- 
scribe its telescopic appearance. How is an atmosphere indicated ? 



SAT UR X. 95 

viewed with a good telescope, appears to be encompassed 
with dusky belts ; but they are far more indistinct than 
those of Jupiter; and instead of crossing the disk in 
straight lines like those of that body, they generally 
present a curved appearance, — an indication of the axial 
inclination. 

Sir William Herscliel inferred the existence of a dense atmos- 
phere surrounding Saturn, both from the changes constantly occur- 
ring in the number and appearance of the belts, and the appearance 
of the satellites at the occurrence of occupations. The nearest was 
observed to cling to the edge of the disk about twenty minutes 
longer than would have been possible had there been no atmos- 
phere to refract the light. Indications of accumulations of ice and 
snow at the poles have also been detected, similar to those of Mars. 

141. Rings. — Saturn is encompassed by three or more 
thin, flat rings, all situated exactly or very nearly in the 
plane of its equator. Two of these rings are very dis- 
tinctly observed, and are designated the interior and the 
exterior rhvj. The former is about 16,500 miles wide ; 
the latter, 10,000 miles. The distance of the interior 
ring from the planet is about IS, '350 miles ; and the 
interval between these rings, about 1,700 miles. The 
thickness of the rings does not exceed 250 miles, and 
may be much less. 

Within the interior ring there is a dusky or semi-trans- 
parent ring, having a crape-like appearance as it stretches 
across the bright disk of the planet. 

Discovery of the Rings.— In his first telescopic examination of 
this planet. Galileo noticed something peculiar in its form. As 

141. How many rings encompass it ? Describe them. What is said of the discov- 
ery of these rings ? By whom was the dark ring discovered ? 



Mb SATURN. 

seen through his imperfect instrument, it appeared to him to have 
a small planet on each side ; and hence he announced to Kepler 
the curious discovery that " Saturn was threefold ;" but continuing 
his observations, he saw, to his great astonishment, these companion 
bodies (as he thought) grow less and less, and finally disappear. 
For fifty years afterward the true ^ause of the appearance remained 
unknown, the distortion of the planet's form being supposed to 
arise from two handles attached to it. Hence they were called 
ansce, the Latin word for handles. Huyghens, in 1659, discovered 
the real cause of the phenomenon, and announced it in these words : 
" The planet is surrounded by a slender, fiat ring, everywhere dis- 
tinct from its surface, and inclined to the ecliptic." The division 
of the ring into two was discovered by an English astronomer in 
1665. Y^e have now certain knowledge of the existence of three 
rings, and some indications of several more. The dark ring was 
discovered by the American astronomer, G. P. Bond, in 1850. 




TELESCOPIC VIEW OF SATURN. - 



The accompanying figure represents the planet as seen through a 
very powerful telescope, showing the dark ring, the division 
between the two main rings, and the line in the outer ring, which 
indicates the existence of an additional ring. 



SATuny. 07, 

142. These rings rotate about the planet on an axis 
perpendicular to their plane, the time of a rotation being 
about 101 hours. As the planet revolves around the sun 
the rings constantly remain parallel to themselves. 




SATURN IN DIFFERENT PAKTS OF ITS ORBIT. 

The accompanying figure represents Saturn in different parts of 
its orbit, the direction of the axis and the position of the plane of the 
rings constantly remaining the same. At A or E, the time of the 
planet's equinox, the plane of the rings passes through the sun, so 
that its edge only is illuminated, wherever the earth may be situated, 
■which is to be conceived as revolving in a small orbit within that 
of Saturn. At the solstice C, the southern side of the rings is 
exhibited ; and at G, the northern side. At the intermediate points 
the rings are viewed obliquely. 

143. Satellitss. — Saturn is attended by eight satel- 
lites, seven of which revolve very nearly in the plane of 
its equator, — the orbit of the eighth, or most distant sateL 
lite, making with that plane an angle of about 12 degrees. 
Their names in the order of their distances from Saturn 
are Jli'mas, EnceVadus, Te'thys, Dio'ne, Rhe'a, Ti'tan, 
Ilype'rioi}, and Jap'etus. 

142. How do the rings rotate ? 143. By what satellites is Saturn attended ? Their 
names ? 

13 



98 



URA NUS. 



144. The following are their periods and distances 
from the primary : 



Distances. 



1. Mimas 

2. ENCELADrS 

3. Tethts 

4. Dione 



22Jh 
Id Gh 

Id 21b 
2d ICh 



121,000 
155,000 
191,000 
246.000 



5. Rhea 

6. Titan 

7. Htperion 

8. JAPETU8 | 



41 i2;h 

15d 23h 

21d Ih 

79d 8h 



343.00D 

796.000 

1,006,000 

2,313,000 



145. The largest of the satellites is Titan, its diameter 
being 3,300 miles, which is larger than that of Mercury. 
The diameters of the others are very much less. 



That of Japetus is 
Tethys and Dione, 500 



1,800 miles; Rbea, 1,200; Mimas, 1,000; 
Enceladus and Hyperion, unknown. 



146. The celestial phenomena at Saturn must present 
a scene of extreme beauty and grandeur. The starry 
vault, besides being diversified by so many satellites, pre- 
senting every variety of phase, must be spanned, in cer- 
tain parts of the planet, and during different portions of 
its long year, by broad, luminous arches, extending to dif- 
ferent elevations, according to the place of the observer, 
and receiving upon their central parts the shadow of 
the planet. 

URANUS tf. 

147. Uranus was discovered in 1781 by Sir William 
Herschel. It shines with a pale and faint light, and to 
the unassisted eye is scarcely distinguishable from the 
smallest of the visible stars. 

144. Their distances, etc. ? 145. Which is the largest of the satellites ? Its 
diameter ? The diameters of the others ? 146. Describe the celestial phenomena 
at Saturn. 147. By whom was Uranus discovered? Its appearance? History of 
its discovery ? 



TTBANUS. 99 

History of its Discovery — This planet had been observed by 
several astronomers previous to its discovery by Herschel, but had 
been mapped as a star at least twenty times between 1690 and 1771, 
its planetary character not having been discerned ; and even Her- 
schel, on noticing that its appearance was different from that of a 
star, was not aware that he had discovered a new placet, but sup- 
posed it to be a comet, and so announced it to the world, April 
19th, 1781. It was, however, in a few months, evident that the 
body was moving in an orbit much too circular for a comet ; but 
its planetary character was not fully established until 1783, when 
Laplace partly calculated the elements of its orbit. 

148. Name and Sign. — It was called by its discoverer 
Georgium Sidus (George's Star) ; but for some time was 
designated Herschel, after its discoverer. Its present 
name U'ranus (in the Grecian mythology the oldest of the 
deities) harmonizes with the names of the other planets. 
Its sign is the letter H with a suspended orb. 

149. Orbit. — The eccentricity of its orbit is about 
82 millions of miles, or about ■£$ of its major axis. Its 
inclination is very small, being about J of a degree. 
Nevertheless, so vast is its distance that, at its greatest 
latitude, it departs more than 24 millions of miles from 
the plane of the ecliptic. 

150. Rotation. — As the disk of Uranus presents neither 
belts nor spots, the period of its rotation and its axial 
inclination still remain unknown. It is thought, from 
the positions of the orbits of the satellites, that the incli- 
nation of its axis is very great ; and analogy would lead 



148. Its name and sign ? 149. What is the eccentricity of its orbit ? 
tion ? 150. Does it rotate ? 



10 J URANUS. 

us to believe that its diurnal period is nearly the same 
as that of Jupiter or Saturn. 

151. Satellites. — Uranus is known to be attended by 
four satellites, which differ from all the other planets of 
the solar system by revolving in their orbits from east 
to west. 

Their orbits are inclined to the plane of that of the primary at 
an angle of 79° ; but, as their motion is retrograde, it seems prob- 
able that the poles have been reversed in position, the south pole 
being north of the ecliptic, and vies versa. The inclination, on this 
supposition, would be 101°. 

NEPTUNE f . 

152. Neptune is the most distant planet known to 
belong to the solar system. It was first discovered by 
Dr. Galle, at Berlin ; but its existence had been pre- 
dicted, and its position in the heavens very nearly ascer- 
tained by the calculations of M. Leverrier (luh-ver-re-d) 
in France, and by Mr. Adams in England, these calcu- 
lations being based upon certain observed irregularities 
in the motion of Uranus. 

History of its Discovery. — The discovery of this planet was 
one of the proudest achievements of mathematical science in its ap- 
plication to astronomy, and afforded a more striking proof of the 
truth of the great law of universal gravitation than had previously 
been ascertained. After the discovery of Uranus, in 1781, it was 
ascertained that the planet had several times been observed by as- 
tronomers, and its place recorded as a star. These positions of the 
planet could not, however, be reconciled with those recorded after 

151. By what satellites is it attended ? For what are they remarkable ? 152. What 
is said of Neptune ? History of its discovery ? 



XEPTUXE. 101 

its actual discovery ; and observation soon showed that its motion 
was at certain points increased, and at others diminished, by some 
force acting beyond it and in the plane of its orbit. These facts 
suggested the existence of another planet, revolving in an orbit ex- 
terior to that of Uranus, and, according to Bode's law, extending 
nearly twice as far from the sun. Adams and Leverrier almost 
simultaneously undertook to find, by mathematical analysis, where 
this })lanet must be in order to produce these perturbations. The 
former reached the solution of this wonderful problem first, and, in 
October, 1845, after three years of toil, communicated to Mr. Airy, 
Astronomer Royal, the result, pointing out the position of the 
planet and the elements of its orbit. The search for the planet was 
not, however, commenced until Leverrier published the result of 
his labors, which was found to agree so closely with that attained 
by Adams, that astronomers both in France and England prepared 
to construct maps of the part of the heavens indicated, in order to 
detect the planet. 

In this they were anticipated by the Berlin observer, who, being 
informed by Leverrier of the result of his computations, and having 
by a fortunate coincidence just received a newly prepared star-map 
of the 21st hour of right ascension (the part of the heavens desig- 
nated by Leverrier), immediately compared it with the stars, and 
found one of them missing. The observations of the following 
evening, by detecting a retrograde motion of this star, established 
its true character. It was the planet sought for, and, wonderful to 
relate, was found only 52' from the place assigned by Leverrier. 
He had also stated its apparent diameter at 3.3" ; it was found by 
actual measurement to be V. Adams's determination of the place 
of the supposed planet differed from the true place by about 2°. 

153. Name and Sign. — The name selected for this 
planet, Neptune, was, in the ancient mythology, the 
name of the god who was supposed to preside over the 
sea. The sign is the head of a trident — the peculiar 
symbol of that deity. 

.153. Its name and sign ? 



102 NEPTUNE. 

154. Distance.— So vast is the distance of this planet 
from the sun (more than thirty times the earth's dis- 
tance), that only Saturn and Uranus can be seen from 
it, — the latter of these bodies never having an elongation 
of more than 40 degrees. Light requires about four 
hoars to pass from the sun to this remote orb. 

155. Orbit. — The eccentricity of its orbit is about 24 
millions of miles, being considerably less than -^ of its 
mean distance ; and, relatively, only about one-half that 
of the earth. Its inclination is also very small (If °). 

155. Solar Light. — The apparent diameter of the sun 
at Neptune is about equal to that of Venus when at its 
greatest elongation ; but its light is vastly more brilliant, 
being estimated as equal to that of 20,000 stars shining 
at once in the firmament, each equal to Venus when its 
splendor is greatest. 

157. Satellite. — A satellite of this planet was discov- 
ered in 1846, and from observations made about the 
same time, another was suspected to exist, but subse- 
quent observations have not positively detected it. The 
orbit of this satellite is nearly circular, and its motion is 
retrograde. 

158. Are there Planets beyond Neptune? — This is 
a question which we are at present entirely unable to 
answer. Future generations may, with greater resources 
of science and mechanical skill, disclose new marvels in 



154. What is said of its distance ? 155. Its orbit ? 156. How does the sun appear 
at Neptune? What is the light equal to? 157. What is said of its satellite? 
158. Are there planets beyond Neptune ? 



THE MINOR PLANETS, OR ASTEROIDS. 103 

our system, and detect other bodies obedient to the do- 
minion of its great central sun. The nearest of the stars 
is known to be nearly 7,000 times as far from Neptune 
as that body is from the sun; and it is by no means im- 
probable, therefore, that so vast a space should contain 
planetary bodies reached by the solar attraction, but very 
far beyond the sphere of any other central luminary. It 
will require, however, far greater means than we possess 
to bring this to a practical determination. 

THE MINOR PLANETS, OR ASTEROIDS. 

159. The Minor Planets, or Asteroids, are a large 
number of very small bodies revolving around the sun 
between the orbits of Mars and Jupiter. The number 
discovered up to the present time (1870) is 112. 

History of their Discovery. — The existence of so large an in- 
terval between Mars and Jupiter, compared with the relative dis- 
tances of the other planets, for a long time engaged the attention 
and incited the researches of astronomers. Kepler conjectured that 
a planet existed in this part of the system, too small to be detected ; 
and this opinion. received considerable support from the publica- 
tion of Bode's law in 1772. When Uranus was discovered, in 1781, 
and its distance was found to conform to this law, the German 
astronomers became so confident of the truth of this bold conjec- 
ture of Kepler, that, in 1800, they formed, under the leadership of 
Baron de Zach, an association of 24 observers to divide the zodiac 
into sections and make a thorough search for the supposed planet. 
This systematic exploration had, however, been scarcely com- 
menced, when, in 1801, Piazzi, an Italian astronomer, while en- 

159. What are the Minor Planets, or Asteroids ? What is their number ? Give 
the history of their discovery. 



lOJf THE MINOR PLANETS, OR ASTEROIDS. 

gaged in constructing a catalogue of stars, detected a new planet. 
It was called by him Ceres. In the next year, while looking for 
the new planet, Olbers discovered another, which he called Pallas. 
The extreme minuteness of the new planets, and the near ap- 
proach of their orbits at the nodes, led Olbers to suppose that they 
might be the fragments of a much larger planet once revolving in 
this part of the system, and shattered by some extraordinary con- 
vulsion. Believing that other fragments existed, and that they 
must pass near the nodes of those already found, he resolved to 
search carefully in the direction of those points ; but while he was 
thus engaged, Harding, of the observatory of Lilienthal, discovered, 
in 1804, very near one of those points, a third planet, which he 
called Juno. Olbers, still further stimulated by this event to con- 
tinue the investigation which he had commenced, was at length, in 
1807, rewarded by discovering a fourth planet, Vesta, near the 
opposite node. From this date until 1845, no additional discovery 
was made. These small planets were called Asteroids by Herschel, 
from their resemblance, in appearance, to stars. In 1845, M. 
Hencke, an amateur astronomer of Driessen, after a series of obser- 
vations continued for fifteen years with the use of the Berlin star- 
maps, discovered Astrma, the fifth of this singular zone of tele- 
scopic planets. The others have been discovered subsequently. • 
Their names and date of discovery are given in the Appendix. 
[See Table.] 

160. Distance. — The average distance of these planets 
from the sun is about 260 millions of miles. That of the 
nearest (Flora) is about 201 millions; that of the most, 
distant (Sylvia) is nearly 320 millions. The entire width 
of the zone in which they revolve is about 190 millions 
of miles. 

161. Orbits. — The inclination of their orbits is very 
diverse ; more than one-third of the whole have a greater 

160. What is their average distance from the sun ? That of the nearest ? Of the 
most distant? How wide is the zone in which they revolve? 161. How great is 
the inclination of their orbits ? Their eccentricity ? 



THE MINOR PLAXETS, OR ASTEROIDS. 105 

inclination than S°, and consequently extend beyond the 
zodiac. The greatest is that of Pallas, amounting to 
34 j°. The eccentricity of their orbits is equally vari- 
able, the greatest being more than one-third its mean 
distance, while the least is only .004. 

These orbits are not concentric ; but if represented on a plane 
surface, would appear to cross each other, so as to give the idea of 
constant and inevitable collisions. " If," says D' Arrest, of Copen- 
hagen, " these orbits were figured under the form of material rings, 
these rings would be found so entangled, that it would be possible, 
by means of one among them taken at a hazard, to lift up all the 
rest." The orbits do not, however, actually intersect each other, 
because they are situated in different planes ; but some of them 
approach within very short distances of each other — in one case . 
less than the moon's distance from the earth. 

162. Magnitude. — The largest of the Minor Planets is 
Pallas, the diameter of which is variously estimated at 
from 300 to TOO miles. These bodies are generally so 
small that it is quite impossible to measure their appar- 
ent diameters, or to say which is the smallest. The 
brightest of these planets is Vesta ; the faintest, Ata- 
lanta. Yesta, Ceres, and Pallas have been seen with 
the naked eye, having the appearance of very small 
stars. 

163. Periods. — The sidereal period of Flora is 3J 
years ; that of Sylvia is about 6-|- years. The average 
period of the whole is about 4J years. 

164. Origin of the Minor Planets. — The theory of 
Olbers has already been alluded to; it supposes that 

162. What is their magnitude ? 163. What is the period of Flora ? Of Sylvia ? 
The average period ? 164. What is said of the origin of the Minor Planets ? 

14 



106 THE MOON. 

these little planets are the fragments of a much larger 
one, which by an extraordinary catastrophe was, in re- 
mote antiquity, shivered to pieces. This theory, how- 
ever, has not been generally accepted, since it is highly 
improbable, being supported by no analogous facts. 

THE MOON. 

165. The Moon, although one of the smallest bodies 
in the solar system, is, to us, next to the sun, the most 
conspicuous, interesting, and important, on account of 
its close connection with our own planet, and the effects 
which it produces upon it. 

165. Orbit.— The orbit of the moon is elliptical; the 
point nearest to the earth being called the Perigee, and 
the point farthest from it, the Ajioyee. Its mean distance 
from the earth is 238,800 miles ; and it is 26,000 miles 
nearer to us in perigee than in apogee. 

Its eccentricity is, therefore, 13,000 miles, or about .055 of its 
mean distance. This is more than three times as great, in projKn- 
tion, as that of the earth. 

167. The positions of the apogee and perigee in space 
are determined by noticing when the moon's apparent 
diameter is greatest and wl en it is least. Careful obser- 
vations of this kind show that these points shift their 
positions, and that the line of apsides completes a cir- 
cuit from west to east in a little less than nine years 

165. What is said of the moon ? 166. Its orbit ? What is perigee ? Apogee ? 
The moon's mean distance from the earth? Its eccentricity? 167. What change 
takes place in the position of the apogee and perigee ? What is this called 1 



THE MOON. 107 

(8y. 310Jd.). This is called the Progression of the 
Apsides. 

168. The inclination of the moon's orbit to the plane 
of the ecliptic is about 5^° ; consequently, it crosses this 
plane in two points, called the Moon's Nodes. The line 
of nodes revolves from east to icest once in about 18-J- 
years. 

169. Diameter. — The mean apparent diameter of the 
moon is a little more than half of a degree, being about 
the same as that of the sun. From this, its distance 
from the earth having been previously determined, is 
found its real diameter, which is 2,162 miles. Its vol- 
ume is therefore somewhat more than -£$ of the earth's. 

170. Phases of the Moon. — The phases of the moon 
are the different portions of her illuminated surface 
which she presents to the earth as she revolves around 
it. When she first becomes visible in the west, she is 
seen as a slender crescent ; but from evening to evening 
her form expands as her angular distance eastward from 
the sun increases, until when in quadrature, or 90° from 
the sun, half of her disk is visible. When she has de- 
parted so far to the east that she rises just as the sun 
sets, the whole of her disk is seen, and she is said to be 
full. After this she becomes the waning moon, rising 
later and later, and growing less and less, until she may 
be seen in the east as a bright crescent just before sun- 

168. What is the inclination of the moon's orhit ? What are the nodes ? How 
does the line of nodes move? 169. What is the moon's apparent diameter? Its 
real diameter ? Its volume ? 1T0. What are the phases of the moon ? Describe the 
moon's apparent motions and phases. 



108 THE MOON. 

rise. A short time after this she disappears, and then 
becomes visible again in the west. 

171. When the moon is in conjunction, the dark side 
being turned toward us, she is called new moon ; when 
she is in quadrature and shows half of her disk, she is 
called half-moon; when she is in opposition, she is 
called full moon. When she is in quadrature after con- 
junction, she is said to be in her first quarter / when in 
quadrature after opposition, in her last quarter. 

When she is between conjunction and quadrature she 
assumes the crescent form, and is then said to be horned ; 
when she is between opposition and quadrature, she ex- 
hibits more than one-half of her disk, but not the whole, 
and is said to be gibbous. 




MOON HORNED AND GD3BOUS. 

In the annexed figure, the partially darkened circle represents 
the moon ; S, the direction of the sun ; E, the direction of the 
earth on one side of the moon, and E', its direction on the opposite 
side. Then a b will represent the line "which separates the illumi- 
nated from the darkened hemisphere of the moon ; and c d, that 

1T1. When is it new moon ? Full moon? First quarter? Last quarter? When 
is the moon said to be horned ? When gibbous ? 



THE MOON. 109 

which separates the hemisphere turned toward the earth from that 
turned away from it. At E, a c being the only part of the disk 
visible, the moon appears horned ; while at E', b c being visible, 
the form is gibbous. 

172. We can, therefore, find the time of a revolution 
of the moon by observing the phases. If the earth were 
at rest, the time from one new or full moon to the next 
would be exactly the period of a revolution ; but as the 
earth is constantly advancing in her orbit, when the 
moon has completed a revolution, she has to move still 
farther in order to come into the same relative position 
with the earth and sun. The time from one new moon 
to the next is 29^ days. This is the synodic period, and 
is called a synodical month, or lunation. The sidereal 
period is 27 J days. 




In the figure, let A B represent the advance of the earth in its 
orbit, while the moon completes a synodic revolution, that is, 
moves from C, the position of inferior conjunction, till she arrives 
at the same relative position with the sun at E. But when she 
reaches this point, she has completed a sidereal revolution, and has 
also moved from D to E, a distance, it will be seen, equal in angu- 

172. How can the time of a revolution of the moon he found ? What is the synodic 
period ? The sidereal period ? Why is the former longer than the latter ? 



110 THE 310 OX. 

lar measurement to A B ; since the arc A B bears the same propor- 
tion to the earth's orbit that E D does to that of the moon. 

173. Owing to the constant advance of the moon in 
her orbit, she rises and, of course, arrives at the meridian 
and sets, about 50 minutes later each successive day. 

This is the average interval of time between the successive 
risings of the moon ; for since she moves through the ecliptic in 
29^ days, her daily advance is equal to about 12£° ; but a place 
upon the earth's surface moves 15° in one hour, and hence requires 
nearly 50 minutes to overtake the moon. If the moon's orbit or 
the eclij)tic, since the inclination is very small, always made the 
same angle with the horizon, this would be the constant interval ; 
but, in consequence of the obliquity of the ecliptic, this angle con- 
tinually varies during each lunation. 

174. Harvest Moon. — This is the full moon that 
occurs in high latitudes, near the time of the autumnal 
equinox, in September and October, when she rises but a 
little later for several successive evenings, and thus 
affords light for collecting the harvest. 

By means of the globe, it may be easily shown that the ecliptic 
is most oblique to the horizon in the signs Pisces and Aries, and 
least so in Virgo and Libra ; so that when the moon is in the 
former signs, in this latitude, she rises only about half an hour 
later, but when in the latter, mora than an hour. This difference 
is, however, only noticed when the moon happens to be full while 
in Pisces or Aries, and thus rises, for several evenings, in the higher 
latitudes, but a few minutes later. These full moons must occur, 
of course, in September and October, when the sun is in the oppo- 
site signs, Virgo and Libra. In the former month, the full moon in 



173. How much later does the moon rise each day? Why ? Why is it not always 
the same ? 174. What is harvest moon ? 



THE MO ON. 



Ill 




HARVEST MOON. 



England is called the Hardest Moon ; in the latter, sometimes, the 
Hunter* & Moon. 

Let H 8HM represent the 
horizon ; S, the position of the 
sun at sunset ; M, the full moon 
just rising ; SAM, the part of 
the equator, and S B M, the 
part of the ecliptic above the 
horizon, the sun being in Libra, 
the autumnal equinox, and the 
moon in Aries, the vernal equi- 
nox. Since the southern half 
of the ecliptic lies east of Libra, 
it will be evident that in or 
near this position the ecliptic 
must make the smallest angle with the horizon ; and consequently, 
while the moon makes her daily advance in her orbit, M b, she 
only descends below the horizon a distance equal to h b ; while, if 
her orbit made a greater angle with the horizon, as S A M, she 
would, by advancing through the equal arc M a, descend below 
the horizon a distance equal to h a. 

175. Observations with the telescope show that the 
moon always presents very nearly the same hemisphere 
to the earth. This proves that it rotates on its axis once 
during each sidereal month, or 27|- days. 

The unassisted eye is able easily to perceive that the dusky spots 
on the disk of the moon constantly keep in the same relative posi- 
tion and present the same appearance ; and this could not occur if 
she rotated so as to present in succession different hemispheres to 
the earth. Just as we infer a rotation of the sun from the appar- 
ent motion of the solar spots, so we know that the moon rotates 
during one revolution around the earth, by the observed fact that 



175. What does an observation of the moon show ? What does this prove ? 
plain this. 



112 THE 310 ON. 

the lunar spots have no apparent motion ; since, if the moon per- 
formed no rotation, the spots on its disk would move across it 
from west to east, keeping pace with the moon's motion in the 
ecliptic, and completing one aj)parent revolution in 29|- days. 

176. Librations of the Moon. — As the orbit of the 
moon is elliptical, her velocity is not uniform — sometimes 
exceeding that of her rotation, and at other times ex- 
ceeded by it In consequence of this, a small portion of 
the hemisphere turned away from the earth becomes 
visible alternately at the eastern and western limbs. 
This is called the lihration in longitude. A portion of 
her surface is also exhibited alternately at each pole, 
caused by the inclination of her axis to the plane of her 
orbit. This is called the lihration in latitude. 

177. Seasons. — Owing to the small inclination of the 
moon's axis to the plane of the ecliptic (1° 31'), she car 
have but very little change of seasons, and that not con 
stant, because her axis does not always point in the sam<t 
direction ; for the lunar solstices and equinoxes chang \ 
places with each other every 9 J years. 

178. Telescopic Appearances of the Moon. — The 

moon's disk when view r ed through a telescope presents a 
diversified appearance of dusky and bright spots; the 
latter being evidently elevated portions of the surface, 
and the former, plains or valleys. Mountains are indi- 
cated by the bright spots that appear scattered over the 



176. What are librations? How is the lihration in longitude cruised? The li- 
hration in latitude? 177. What seasons has the moon? Why? 178. What does 
the moon's disk exhibit when viewed through a telescope? How are mountains 
indicated ? Of what form are they ? 



THE MOOm 113 

disk and beyond the terminator, and by the shadows 
cast upon the surface of the moon when the sun shines 
obliquely upon these elevations. These mountains are 




photogkaphic views of the moon.— De La Eve, 

of various forms, many of them being very lofty, of a 
circular form, and inclosing an extensive area, sometimes 
CO miles in diameter. These are called Ring Moun- 
tains. Others, called Crater Mountains, inclose hollow 
spaces, which often contain a central mound. Some of 
the lunar mountains are evidently over 20,030 feet high. 

179. Appearances indicate that the moon has very 
little, if any, atmosphere ; and that its surface is as devoid 
of water as of air. 

When viewed with a telescope, the surface of the moon appears 
entirely unobscured by any clouds or vapors floating over it ; and 
when the moon's edge comes in contact with a star, the latter is 

179. Has the moon an atmosphere ? How is this indicated ? 

15 



11£ THE 310 ON. 

immediately extinguished ; whereas, if there were an atmosphere, 
it would, from the effect of refraction, rest on the edge for a short 
time ; that is, it would be visible when a short distance actually 
behind the moon. Observations of this kind have been made with 
so much nicety, that it is believed that an atmosphere two thousand 
times less dense than that of the earth could not have escaped 
detection. If any atmosphere therefore exists, it must be rarer 
than the attenuated air in the exhausted receiver of the most per- 
fect air-pump. 

180. Are there People in the Moon? — This ques- 
tion has often been discussed, but idly ; since no positive 
evidence can be adduced on one side or the other. The 
distance of the moon is too great for us to detect any 
artificial structures, as buildings, walls, roads, etc., if 
there were any ; and certainly, without air or water, no 
animals such as inhabit our own planet could exist there. 
But the Almighty Creator can place animals aucl intelli- 
gent beings in any part of the universe, and accommo- 
date them to the peculiar circumstances of their abode ; 
and it would perhaps be strange if He had left even our 
little satellite without an intelligent witness of His infi- 
nite power and beneficence. 

180. Are there people in the moon ? 



SECTION V. 



ECLIPSES. 

181. An Eclipse is the concealment or obscuration 
of the disk of the sun or moon by an interception of the 
sun's rays. Eclipses are, therefore, either Solar or Lunar. 

A solar eclipse is caused by the passage of the moon 
between the earth and sun so as to conceal the sun from 
our view. 

A lunar eclipse is caused by the passage of the moon 
through the earth's shadow. 

By a shadow is meant simply the space from which the light of 
a luminous body is wholly intercepted by the interposition of some 
opaque body. Since light proceeds from a luminous body in straight 
lines, and in all directions, the darkened space formed behind the 
earth or moon must be conical ; that is, of the form of a cone, cir- 
cular at the base and terminating at a point ; since the sun or 




181. What is an eclipse ? Of how many kinds are eclipses ? What is a solar 
eclipse ? A lunar eclipse ? What is meant by the shadow ? Its form ? What is it 
sometimes called ? What is the penumbra ? 



116 ECLIPSES. 

luminous body is larger than either of the opaque bodies. The 
shadow is sometimes called by its Latin name, umbra. Besides the 
totally darkened space called the umbra, there is formed on each 
side a space from which the light is only partially excluded ; this 
is called the penumbra. The relations of the umbra to the penum- 
bra will be understood by inspecting the annexed diagram. 

182. If the moon moved exactly in the plane of the 
earth's orbit, a solar eclipse would occnr at every new 
moon, and a lunar eclipse at every full moon ; but as the 
moon's orbit is inclined to that of the earth, an eclipse 
can only happen when the moon is at or near one of its 
nodes. 




AVhen the moon is new or full at a considerable distance from 
its node, it is too far above or too far below the plane of the eclip- 
tic to intercept the sun's rays from the earth, or to pass within the 
limits of the earth's shadow. It will be easily understood that no 
eclipse can occur unless the sun, earth, and moon are situated 
exactly or nearly in the same straight line. [See Figure.] 

183. The distance, either side of the node, within 
which an eclipse can occur, is called the Ecliptic Limit. 
The solar ecliptic limit extends about 17° on eacli side 

182. Why does not an eclipse occur at every new and full moon ? 183. What is 
the ecliptic limit ? The extent of the solar ecliptic limit ? The lunar ecliptic limit ? 
Which eclipses are the more frequent ? Why ? What is the greatest numher of 
eclipses that can happen in a year ? The least numher ? 



ECLIPSES. 117 

of the node ; the lunar ecliptic limit, only 12°. Eclipses 
of the sun are, therefore, more frequent than those of 
the moon. The greatest number of eclipses that can 
happen in a year is seven, — five of the sun and two of 
the moon, or four of the sun and three of the moon. 
The least number is two, both of which must be of 
the sun. 

184. Solar eclipses do not actually occur as often as 
lunar eclipses at any particular place ■• because the lat- 
ter are always visible to an entire hemisphere, whereas 
the former are only visible to that part of the earth's sur- 
face covered by the moon's shadow or its penumbra. 

185. When the whole of the sun's or moon's disk is 
concealed, the eclipse is said to be total ; when only a 
part of it is concealed, it is said to he partial. 

In order to measure the extent of the eclipse, the 
apparent diameters of the sun and moon are divided into 
twelve equal parts, called Digits. 




A PARTIAL ECLIPSE OF THE SUN AND MOON. 

The conditions of a total and a partial eclipse will be apparent 
from the explanations already given. VThen the centres of the sun 

184. Which are the most frequent at any place ? Why ? 185. What is a total 
eclipse ? A partial eclipse ? What are digits ? 185. What is an annular eclipse ? 



118 



ECLIPSES. 



and moon coincide, that is, when the latter is exactly at the node, 
the eclipse is said to be central. A central eclipse of the moon 
must, of course, be total ; but a solar eclipse may be central with- 
out being total ; since sometimes the shadow of the moon does not 
reach the earth. The moon, when this is the case, covers only 
the central part of the sun's disk, leaving a ring of luminous sur- 
face around the opaque body. 

185. An annular eclipse is an eclipse of the sun, which 
happens when the moon is too far from the earth to con- 
ceal the whole of the sun's disk, leaving a bright ring 
around the dark body of the moon. 




AN ANNULAR ECLIPSE. 

187. The phenomena connected with a total eclipse 
of the sun are of a peculiarly interesting character, and 
have been observed by astronomers with great attention 
and industry. 

To an ignorant mind, this occurrence must be the occasion of 
very great awe, if not actual terror. A universal gloom overspreads 
the face of the earth as the great luminary of day appears to be 
expiring in the sky ; the stars and planets become visible, and the 
animal creation give signs of terror at the dismal and alarming aspect 
of nature. Armies about to engage in battle have thrown down 
their arms and fled in dismay from the seeming anger of heaven. 

186. What is an annular eclipse ? 187. Describe the phenomena connected with 
a total eclipse of the sun. 



ECLIP SE S. 110 

188. An oceultation is the concealment of a planet or 
star by the interposition of the moon or some other body. 

The oceultation of a planet or star by the moon is a very inter- 
esting and beautiful phenomenon. From new moon to full moon, 
she advances eastward with the dark edge foremost, so that the 
occulted body disappears at the dark edge and re-appears at the 
enlightened edge. In the other part of her orbit this is reversed. 
The former phenomenon is of course the more striking, the star or 
planet appearing to be extinguished of itself. 

189. Transits. — When the latitude of Mercury or 
Venus, at the time of inferior conjunction, is less than 
the semi-diameter of the sun, a transit must occur, the 
planet appearing on the sun's disk like a small, round, 
and intensely black spot, and moving across it from east 
to west. 

It appears to move across the disk from east to west for the same 
reason that the solar spots appear to move in that direction (Art. 
106). The planet's velocity being faster than the earth's, the planet 
passes the earth actually from west to east, but in a direction oppo- 
site to the diurnal motion of that part of the earth on which the 
observer stands ; hence the apparent motion is westward, since east 
is in the direction of the rising sun, and this must be the point 
toward which any place on the earth turns. 

190. The transits of Venus are of great interest and 
importance, because they afford a means of determining 
the distance of the sun from the earth. The next transit 
will occur in December, IS 74. 

The transits of Jupiter's satellites are among the most 
interesting celestial phenomena. 

188. What is an oceultation? Describe the phenomenon. 189. When does a tran- 
sit of Xercury or Venus occur ? Describe the phenomenon. 190. Why are these 
transits of great interest ? 



SECTION VI. 



191. Tides are the alternate rising and falling of the 
water in the ocean, bays, rivers, etc., occurring twice in 
about twenty-four hours. The rising of the water is 
called Flood Tide • the falling, Ebb Tide. They are 
caused by the unequal attraction of the sun and moon 
upon the opposite sides of the earth. 

Since the attraction of gravitation varies inversely as the square 
of the distance, the sun and moon must attract the water on the 
side nearest to them more than the solid mass of the earth ; while 
on the side farthest from them, the water must be attracted less 
than the solid earth ; hence, there must be a tendency in the water 
to rise at each of these points ; it being drawn away from the earth 
at the point toward the sun or moon, and the earth being drawn 
away from it at the other point. At the points 90° from these, the 
effect of the attraction is just the reverse ; for since it does not act 
in parallel lines, it tends to draw together the two sides of the 
earth, and thus congresses the water so as to cause it to recede, 
and hence increases its tendency to rise at the other points. 

Thus at A [see diagram, page 121] the water is attracted by the sun 
more than the earth, and at B less ; while at C and D the attraction 
squeezes in the water, as it were, so as to make it recede still more, 
and thus to augment its rising at A and B. It will be obvious that 
the attraction of the moon acts so as 'to disturb the water just as 
that of the sun does ; hence, in the position represented in the dia- 

191. What are tide? ? What is flood tide ? Ehh tide ? How are tides caused ? 
Explain by the diagram. 



TIDES. 121 

gram, when the moon is in opposition, the action of both the sun 
and moon is exerted upon the same points, A B and C D ; and it 

c 

Mi 



5 1 l^w-it 3 



will also be obvious that if the moon were in conjunction, that is, 
on the same side of the earth as the sun, the effect would be the 
same, because the same points would be acted upon. 

192. Similar tides, therefore, occur simultaneously on 
opposite sides of the earth ; namely, flood tides on the 
side turned toward and on that turned from the sun or 
moon, and ebb tides at the two points 90 degrees distant. 

193. Since the moon is so much nearer to the earth 
than the sun is, its attraction on the opposite sides is 
much more unequal, and consequently its disturbing 
action is greater. At the mean distance of the sun and 
moon from the earth, the disturbing or tidal force of the 
moon is about 2^ times that of the sun. 

194. When the sun and moon are on the same or 
opposite sides of the earth, they unite their attractions, 
and thus raise the highest flood tides at the points under 
or opposite them, and the lowest ebb tides at the points 
90° from these. Such tides are called Spring Tides; 

192. When do similar tides occur ? 193. Which has the greater disturhing force — 
the sun or the moon ? Why ? 194. What are spring tides ? How are they caused ? 
When do they occur ? 

16 



they occur at every new and full moon, or a short time 
afterward. 

195. When the moon is in quadrature, its tidal force 
is partly counteracted by that of the sun, since the two 
forces act at right angles with each other ; and conse- 
quently the water neither rises so high at flood, nor 
descends so low at ebb tide. Such tides are called JVeap 
Tides; they occur when the moon is in either of the 
quarters. 




The annexed diagram represents neap tide. The effect of the 
sun at A and B, and of the moon at C and D, is to equalize the 
height of the water all over the earth. The pupil must understand 
in inspecting these diagrams, that the actual effect of the sun or 
moon is not, by any means, so great as is represented. It is, in fact, 
but a few feet, while the earth's diameter is nearly 8,000 miles. 

196. In the northern hemisphere, the highest tides 
occur during the day in summer, and during the night 
In winter. In the southern hemisphere the reverse of 
this is the case. 

195. What are neap tides ? How caused ? When do they occur ? Explain hy the 
diagram. 196. When do the highest tides occur in the northern hemisphere? In 
the southern hemisphere ? Explain hy the diagram. 



TIDES. 128 

This will be apparent from an inspection of the diagram on 
page 121. The greatest tidal elevation of the water is of course at 
A and B, and diminishes north and south of these points. At a 
the elevation of the water is obviously greater than at ~b ; but a is 
the position of a place in the northern hemisphere at noon and in 
summer, since the north pole is turned toward the sun, and &, its 
position at midnight ; so that the tide is higher during the day than 
during the night, in this season. Conceive the pole turned the other 
way, and it will be at once seen that the reverse is true in winter. 

197. The tides do not rise at the same hour every day, 
but generally about 50 minutes later ; because, as the 
moon advances in her orbit, the same place on the earth's 
surface does not come again under the moon until about 
50 minutes later than on the previous day. 

The interval which elapses from the moon's passing the meridian 
of a place until it returns to the same again is generally 24h. 50m. 
28s. ; the interval, therefore, between two successive tides is 12h. 
2om. 14s. 

198. The tide does not generally rise until two or 
three hours after the moon has passed the meridian ; 
because, on account of its inertia, the water does not 
immediately yield to the action of the sun or moon. 

By inertia is meant the resistance which matter of every kind 
makes to a change of state, whether of rest or motion ; that is, it 
cannot put itself in motion, neither can it stop itself. The tides 
are not only retarded by inertia, but, to some extent, by the friction 
on the bed of the ocean or the sea, or the sides of rivers and con- 
fined channels. 

199. The tides that occur in rivers, narrow bays, or 

197. How much, later do the tides rise from day to day? Why? "What is the 
interval between two successive tides ? 198. Why does not the tide rise when the 
moon is on the meridian ? What is inertia ? 199. How are the tides of rivers, etc., 
caused ? What are primitive tides ? Derivative tides ? 



124 TIDES. 

other bodies of water at a distance from the ocean, are 
not caused by the immediate action of the sun and moon, 
but arise from the movement of the great ocean tide 
wave, urging the water into these contracted inlets. The 
tide 5 of the ocean are called Primitive Tides j those of 
rivers, inlets, etc., are called Derivative Tides. 

200. The average height of the tide for the whole 
globe is about 2^- feet ; and this is the height to which 
it rises in the ocean. The height, however, at any par- 
ticular place depends upon its situation ; the highest 
tides occurring in narrow bays, and arms of the sea run- 
ning up into the land. Lakes have no perceptible tides. 

The highest tides in the world take place in the Bay of Fundy, 
the mouth of which is exposed to the great Atlantic tide wave. At 
the head of the bay the ordinary spring tides rise to the height of 
50 feet, while sj^ecial tides have been known to rise as high as 
70 feet. In New York, the height of the tide is, at its maximum, 
about 8 feet ; in Boston, 11 feet. On the other hand, at some 
places there is scarcely any tide at all. An instance of this is found 
at a point on the south-eastern coast of Ireland, the tide stream 
being diverted to the opposite shore by a promontory at the 
entrance of St. George's Channel. 

200. What is the average height of the tide for the whole glohe ? Why does it 
vary at different places ? When are the tides highest ? Tides in lakes ? Mention 
the height at some particular places. 



SECTION VII. 

COMETS. 

201. Comets are bodies of a nebulous or cloudy 
appearance that revolve in very eccentric orbits, and are 
generally accompanied by a long and luminous train 
called the Tail. The head of the comet consists almost 
always of a bright and apparently solid part in the cen- 
tre, called the Nucleus, and a nebulous substance, called 
the Coma, which envelops it. The tail extends on the 
side from the sun. 

The name comet is derived from this nebulous appearance which 
the ancients fancifully likened to hair [in the Greek, come], and 
hence called these bodies cometm or hairy bodies. When the lumi- 
nous train precedes the comet, it is sometimes called the beard. 

The apj)earance of comets is not uniform, the same comet chang- 
ing very much at different times. Some comets have no nucleus, 
others, no tails ; while still others have several tails. 

These bodies when at a long distance from the earth and sun are 
distinguished from planets by the size and position of their orbits, 
and the direction of their motions. Uranus was for some time 
thought to be a comet, and was recognized as a planetary body 
only after its orbit had been proved to be almost circular, and 
nearly in the plane of the ecliptic. 

201. What are comets ? Of what does the head of a comet consist ? On which 
side is the tail ? From what is the name comet derived ? Appearance of comets ? 
How distinguished from planets ? 



126 



COMETS. 



202. Comets either revolve around the sun in ellipti- 
cal orbits, or move in curve lines of other forms. The 
elliptic comets may be considered as belonging to the 
solar system ; the others, as only visitants of it, since 
they come from distant regions of space, move around 
one side of the sun, and then pass swiftly away in paths 
that never return into themselves, but are constantly 
divergent. 




ORBITS OF COMETS. 



In the diagram these different kinds of paths are represented. 
The smaller one is an elliptic orbit of very great eccentricity, A 
being the aphelion, and P the perihelion, a P b and c P d are 
paths that do not return into themselves. The former is a curve, 
called by mathematicians a parah'ola ; the latter is called a hypcr'- 
bola. The greater divergency of the latter will be obvious ; also, 
that the elliptic and parabolic curves coincide from 1 to 2, and 



202. What is the shape of comets 1 orhits ? To what do elliptic comets belong ? 
Other comets ? Explain the different kinds of orbits. 



C ME TS. 127. 

thus that these two orbits would be undistinguishable between 
these two points. The motion indicated by the arrows is direct. 

203. In order to identify a comet, or ascertain that it 
is the same which has previously appeared, we mnst 
know, 1. The longitude of the perihelion of its orbit ; 
2. The longitude of its ascending node; 3. Its inclination 
to the ecliptic ; 4. Its eccentricity; 5. The direction of 
the comet's motion ; and 0. Its perihelion distance from 
the sun. These facts are called the Elements of its 
Orbit. 

204. Elliptic Comets. — The elliptic comets are di- 
vided into two classes ; those of short periods and those of 
long periods. The former are seven in number, and have 
all reappeared several times, their identity being satis- 
factorily established by an entire correspondence of their 
elements. The most noted of these is the comet of 
Encke, the period of which is about 3^ years, nineteen 
returns of it having been recorded. 

The others are Be Two's, the period of which is 5-|- years ; Win- 
necJce's, 5^ years ; Brorseri's, 5f years ; Biela's, 6f years ; B'Arrestfs, 
6 1 years; Fayeh, 7-§- years. These cornets are named after the dis- 
tinguished astronomers who first discovered them, or determined 
their periods and predicted their returns. 

205. These comets have comparatively small orbits, 
the mean distance of each being less than that of Jupi- 
ter, and all revolving within the orbit of Saturn. The 



203. How is a comet identified? What are the elements of a comet's orbit? 
234. How are the elliptic comets divided ? How many comets of short period are 
there ? Which is the most noted ? Its period and returns ? 205. What is said of 
this class of comets ? 



128 COMETS. 

inclination of their orbits is comparatively small ; and 
they all revolve from west to east. They are not con- 
spicuous objects, but have been generally visible only 
with the aid of a telescope. 

206. With the exception of a few comets, the period] 
of which have been computed to be about 75 years, all 
the remaining elliptic comets are thought to be of ven 
long periods, some more than 100,000 years. 

The comet of 1744 is estimated to require nearly 123,000 years 
to complete one revolution ; that of 1844, 102,000 years; and the 
great comet of 1680, about 9,000 years. The period of a comet 
cannot, however, be ascertained with precision during one appear- 
ance, since only a very small part of its orbit is described during 
the short time it remains visible. There is, consequently, consid- 
erable uncertainty in these determinations. To the great comet of 
1811, the two periods of 2,301 and 3,065 years have been assigned. 

237. Orbits of Comets.— Of all the comets whose 
orbits have been ascertained, about one-half are direct, 
that is, they revolve from west to east ; the remainder arc 
retrograde. Their inclinations are very diverse, some re- 
volving within the zodiac, others at right angles with the 
ecliptic, 

About three-fourths of all the comets have their perihelia within 
the orbit of the earth ; and nearly all the others, within the orbit 
of the nearest asteroid. Only one is situated more than 400,000,000 
miles from the sun. Some comets, on the other hand, come into 
close proximity to the sun. The great comet of 1680 approached 
within 600,000 miles of it ; and that of 1843 was less than 75,000 
miles. The aphelion distances of some of these comets are incon- 
ceivably great. The comet of 1811 recedes to a distance from the 



206. Of the other elliptic comete ? Give some examples. 207. In what direction 
do comets move ? The inclination of their orbits ? 



COMETS. 129 

sun equal to 14 times that of Neptune, or more than 40,000 millions 
of miles; the greatest known (that of 1844) must be nearly 400,000 
millions of miles. 

208. The Velocity of Comets as they move through 
their perihelia is amazingly great. That of 1680 was 
880,000 miles an hour; and that of 1843, about 1,260,000 
miles an hour, or 350 miles per second. The latter body 
swept around the sun from one side to the other in about 
two hours. 

209. The Number of Comets is supposed to be very 
great. From the earliest period up to the present time 
more than 800 have been recorded, of which nearly 300 
have had their orbits computed, and of the latter 54 have 
been identified as returns of previous comets. 

Since it is only within the last 100 years that optical aid has 
been made available in searching for comets, it is supposed that 
the actual number of comets that have come within view, in both 
hemispheres, is not less than 4,000 or 5,000. M. Arago estimates 
that the greatest possible number in the solar system cannot exceed 
350,000. 

210. The Size of Comets, including both envelope 
and nucleus, very much exceeds that of the largest 
planet ; the nucleus is, however, comparatively small, 
the diameter of the largest measured being about 8,000 
miles (that of 1845). 

211. The Masses and Densities of the comets must 
be inconceivably small; since, notwithstanding their 

208. The orbital velocity of comets ? 209. The number of comets ? 210. Their 
6lze? 211. Masses and densities ? Eemark? 

17 



ISO COMETS. 

great magnitudes, they move among the planets and 
their satellites without in the least, as far as it can be 
observed, affecting their motions ; although they are 
themselves greatly disturbed by the attractions of the 
planets. 

Their densities are, without doubt, many thousand times less 
than atmospheric air. Stars are seen very clearly through the 
nebulous coma and train of a comet, notwithstanding that the light 
has to pass sometimes through millions of miles of the substance. 

212. The Tails of comets are often of immense length, 
and are generally of a bent or curved form, extending on 
the side from the sun and nearly in a line with the 
radius-vector of the orbit. The tail increases in length 
as the comet approaches the sun, but attains its greatest 
dimensions a short time after the perihelion passage, and 
then gradually diminishes. 

In respect to magnitude, the tails of comets are the most stu- 
pendous objects which the discoveries of astronomers have pre- 
sented to our contemplation. That of the comet of 1680 was more 
than 100,000,000 miles in length ; while the comet of 1843 pre- 
sented a train 200,000,000 miles long, which was shot forth from 
the head of the comet in the incredibly short space of twenty clays. 
The increase of the tail and the decrease of the head of the comet 
as it ap p roaches the sun, are among the most striking phenomena 
presented by these bodies. 

REMARKABLE COMETS. 

213. Comet of 1680. — This was the comet that New- 
ton subjected to the calculations by which he showed 

212. Their tails ? Remark ? 213. What account is given of the comet of 1680 ? 



COMETS. 



131 



that these bodies revolve in one of the conic sections, 
and that they are retained in their orbits by the same 




GKEAT COMET OF 



force that binds the planets to the sun. It was very 
remarkable for its splendor, and for the extent of its 
train, which stretched over an arc of 70° in the heavens, 
and reached the amazing length of 120,000,000 miles. 
With the exception of the comet of 1843, it approached 
nearer to the sun than any other known, and moved 
through its perihelion with a velocity of 880,000 miles 
an hour. 

214. Halley's Comet. 

This comet derives its name 
from Sir Edmund Halley, a 
celebrated English astrono- 
mer, who calculated its orbit 
and predicted its return. It 
appeared in 1682, and Hal- 
ley noticing a close resem- 
blance in its elements to 
those of 1531 and 1607, 




t's comet, 1835. 



214. What account is given of Halley's comet ? 



132 



COMETS. 



concluded that the comets of these years were differ- 
ent appearances of the same comet, and ventured to 
predict its reappearance in 1758 or 1759. This predic- 
tion was realized by the return of the comet in March, 
1759 ; and it again appeared in 1835. These different 
appearances, it will be observed, were about 75 years 
apart ; and others of an earlier date have also been 
recognized. 

215. Encke's Comet is remarkable for its short 
period and frequent returns. Its period and elliptic 

orbit were determined by 
Professor Encke at its 
fourth recorded appear- 
ance in 1819. Its last re- 
turn took place in 1868, 
This comet has generally 
appeared without any lumi- 
nous train ; but in 1S48, 
it had a tail about 1° long, 
turned from the sun, and 
a shorter one directed to- 
ward that luminary. In its latest returns it has been 
very faint and difficult of observation. 




ENCKE S COMET. 



216. Lexell's Comet. — This body is particularly 
noted for the amount of disturbance which it has suf- 
fered in passing among the planets. From observations 
made in 1770, Lexell calculated its period to be about 



215. What account is given of Encke's comet ? 216. Of Lexell's comet ? What is 
determined "by calculation in respect to it ? 



COMETS. 



133 



i§ years ; and it was a large and bright object. It bas, 
however, never been seen since, its orbit having, without 
doubt, been entirely changed by planetary disturbance. 

It is proved by calculation that it must have returned in 1776, 
but was so situated as to be continually hid by the sun's rays ; and 
also, that in 1779 it passed so near Jupiter, that its orbit must have 
been greatly enlarged, so that it no longer comes near the earth. 
On July 1st, 1770, its distance from the earth was less than 1,500,000 
miles. 

217. Comet of 1744. 

— This was the finest 
comet of the 18th cen- 
tury, and, according to 
some observers,- had six 
tails spread out in the 
form of a fan. Euler 
calculated its elliptic or- 
bit, and assigned to it a 
period of 122,663 years. 
Its motion was direct. 




COMET OF 1744. 



218. Biela's Comet. — This is one of the elliptic 
comets of short period ; its perihelion lying just within 
the orbit of the earth, and its aphelion a little beyond 
that of Jupiter. The orbit of this body nearly crosses 
the actual path of the earth ; and in 1 832, Olbers cal- 
culated that it would come within 20,000 miles of the 
earth, so that the latter body would be enveloped in its 
mass. The earth, however, did not reach the node until 
one month after the comet had passed it. 



217. What is said of the comet of 1744 ? 218. Of Biela's comet ? 



lSJf. COMETS. 

In 1845, this comet became elongated in form and finally sep- 
arated into two comets, which traveled together for more than 
three months ; their greatest distance apart being about 160,000 
miles. The two parts were again seen at the next return of the 
comet in 1852, but the interval had increased to 1,250,000 miles. 
Neither part has been seen since. 




COMET OF 1811. 

219. Comet of 1811. — This comet was very remark- 
able for its unusual magnitude and splendor. It was 
attentively observed by Sir William Herschel, who de- 
scribes it as having a nucleus 428 miles in diameter, 
which was ruddy in hue, while the nebulous mass sur- 
rounding it was of a bluish-green tinge. Its tail was of 
peculiar form and appearance, extending about 25°, with 
a breadth of nearly 6°. 

The investigation of its elements by Argelander is the most com- 
plete ever made. He assigns it a period of more than 3,000 years, 
and estimates its aphelion distance at 40,121 millions of miles. 

220. Comet of 1843. — This comet was also remark- 
able for its size and brilliancy, it being visible in some 
parts of the world during the day time. It had a tail 

219. What account is given of the comet of 1811 ? 220. Of the comet of 1843 ? 



COMETS. 135 

60° long, and approached within a very short distance of 
the snn, — about 75,000 miles from its surface. Its period 




GREAT COMET OF 1843. 



is variously estimated at from 175 to 376 years. Its 
motion is retrograde. 

221. Donati's Comet. — This is the great comet of 
1858, named after Donati, by whom it was first seen at 
Florence. As it approached its perihelion it attained a 
very great magnitude and splendor, and was particularly 
distinguished for the magnificence of its train. Its period 
has been estimated at nearly 1,900 years. 

222. Recent Comets. — About thirty comets have 
appeared since that of Donati, the elements of which 
'have been calculated. The most remarkable were the 
comet of 1861, described as one of the most magnificent 
on record, having a tail 100° long; and that of 1862, 
which was very interesting for the peculiar phenomena 
which it presented of luminous jets, issuing in a continu- 
ous series from its nucleus. 

221. What account is given of Donati's comet ? 222. Recent comets ? 



SECTION VIII. 

METEORS OR SHOOTING STARS. 

223. Meteors or Shooting Stars are small luminous 
bodies that move rapidly through the atmosphere, fol- 
lowed by trains of light, and quickly vanishing from 
view. They sometimes appear in numbers so great as to 
seem like showers of stars. 

224. These star-showers are found to occur at certain 
periods. Every year, about November 14th, there is a 
larger fall than usual of meteors ; but about every 33 
years, it has been noticed, there is a great star-shower. 
Those which occurred in November, 1866-7, had been 
predicted from observations of previous events of the 
kind. Thus a star-shower occurred in November, 1832-3, 
also in 1799 ; and there are eighteen recorded observa- 
tions of the phenomena from 1698 to 902, all correspond- 
ing in period to that mentioned above. 

Great Star-showers. — The shower of 1799 was awful and sub- 
lime beyond conception. It was witnessed by Humboldt and his 
companion, M. Bonpland, at Cumana. in South America, and is 
thus described by them : — " Toward the morning of the 13th of 
November, 1799, we witnessed a most extraordinary scene of shoot- 
ing meteors. Thousands of bolides and falling stars succeeded each 

223. What are meteors ? 224. At what periods do star-showers occur ? How 
many recorded observations of star-showers are there since 902 ? Give some ex- 
amples of great star-showers. 



METEORS OR SHOOTIXG STARS. 137 

other during four hours. Their direction was very regularly from 
north to south ; and from the beginning of the phenomenon there 
was not a space in the firmament equal in extent to three diam- 
eters of the moon, which was not filled every instant with bolides 
or falling stars. All the meteors left luminous traces, or phosjDho- 
rescent bands behind them, which lasted seven or eight seconds.' 1 
The same phenomena were seen throughout nearly the whole of 
North and South America, and in some parts of Europe. The 
most splendid display of shooting stars on record was that of No- 
vember 13th, 1833, and is especially interesting as having served to 
point out the periodicity in these phenomena. Over the northern 
portion of the American continent the spectacle was of the most 
imposing grandeur; and in many parts of the country the popula- 
tion were terror-stricken at the awfulness of the scene. The igno- 
rant slaves of the southern States supposed that the world was on 
fire, and filled the air with shrieks of horror and cries for mercy. 
The shower of 18G6 was anticipated with great interest ; and in 
New York and other places arrangements were made to announce 
the occurrence, during the night of November 14th, by ringing the 
bells from the watch-towers. The display, however, was not wit- 
nessed in this country, but in England was quite brilliant ; as many 
as 8.000 meteors being counted at the Greenwich observatory. 
Another shower of less extent occurred in November, 1867. 

225. Meteoric Epochs are particular times of the 
year at which large displays of shooting stars have been 
observed to occur at certain intervals. The principal of 
these are November 13th-lttth, and August 6th-llth. 

Three others have been established with considerable certainty ; 
namely, in January, April, and December, and still others indi- 
cated, that are doubtful. There are 56 meteoric days in the year ; 
those in August and November being the richest. 



225. What are meteoric epochs ? Which are the principal ones ? What others are 
there ? Meteoric days ? 

18 



138 METEORS OR SHOOTING STARS. 

226. Meteors are supposed to be small bodies collected 
in rings or clusters, and revolving around the sun in 
eccentric orbits. They appear to resemble comets in 
their nature and origin, and, like those bodies, sometimes 
revolve from east to west. 

227. Origin of Meteors. — The immense velocity of 
these bodies, which is about equal to twice that of the 
earth in its orbit, or 36 miles a second, and the great ele- 
vation at which they become visible, the average being 
60 miles, indicate that they are not of terrestrial, but 
cosmical, origin ; that is, they emanate from the inter- 
planetary regions, and being brought within the sphere 
of the earth's attraction, precipitate themselves upon its 
surface. Moving with so great a velocity through the 
higher regions of the air, they become so intensely heated 
by friction that they ignite, and are either converted into 
vapor, or, when very large, explode and descend to the 
earth's surface as meteoric stones, or aerolites. The bril- 
liancy and color of meteors are variable ; some arc as 
bright as Venus or Jupiter. About two-thirds aro 
white ; the remainder, yellow, orange, or green. Their 
number is inconceivably great. 

228. Fire Balls are large meteors that make their 
appearance at a great height above the earth's surface, 
moving with immense velocity, and accompanied by 
luminous trains. They generally explode with a loud 



226. What are meteors supposed to he ? What do they resemhle ? 227. Of what 
origin are they ? How is this indicated ? How are meteoric stones or aerolites 
produced ? What is said of the brilliancy of meteors ? Their colors ? Their num- 
ber ? 228. What are fire balls ? 



METEORS OR SHOOTING STARS. 139 

noise, and sometimes descend to the earth in large 
masses. There are very remarkable occurrences of this 
kind on record. 

229. The Composition of Aerolites is nearly 
always the same, iron being the principal ingredient. 
Some of these masses are of immense size ; one, a mass 
of iron and nickle, found in Siberia, weighs 1,680 lbs. 
At Buenos Ayres there is a mass partly buried in the 
ground, TJ- feet in length, and supposed to weigh about 
16 tons. 

230. The November Meteor3 are supposed to re- 
volve around the sun in an orbit of considerable eccen- 
tricity, inclined to the plane of the ecliptic in an angle 
of 17^°, and extending at its aphelion somewhat beyond 
the orbit of Uranus, its perihelion being very nearly at 
that of the earth. They move in a ring of unequal 
width and density, the thickest part crossing the earth's 
orbit every 33 years, and requiring nearly two years to 
complete the passage. 

231. The elements of this orbit correspond almost 
precisely with those of the comet which made its appear- 
ance in January, 1866 ; so that it seems probable that 
the comet is a very large meteor of the November 
stream. The elements of the orbit of the August me- 
teors have been found, in a similar manner, to coincide 
with those of the third comet of 1862 ; showing that 



229. What is the composition of aerolites ? 230. What is the orbit of the Novem- 
ber meteors ? How do they move ? 231. To what does this orbit correspond ? 
What is inferred from this ? 



1J/0 METEORS OR SHOOTING STARS. 

the comet and these meteors belong to the same ring. 
This seems also to be true of the first comet of 1861 and 
the April meteors. 

232. The point from which the November meteors 
seem to radiate is in the constellation Leo ; because, as 
the earth at that time of the year is moving toward 
that point, they appear to rush from it. Their velocity 
appears to be double that of the earth, although only 
equal to it ; because they move in an opposite direc- 
tion and almost in the same plane. When the earth 
plunges into the meteoric stream a great star-shower 
occurs. 

233. Physical Origin.— Meteors are supposed by some 
to be small fragments of nebulous matter detached in vast 
numbers from larger masses, such as are seen in the 
regions of the stars, or from that of which the solar sys- 
tem is supposed to have been originally formed, their 
origin being precisely the same as that of the comets, 
which indeed may be considered as, in reality, only 
meteors of vast size. It is also probable that, like 
Biela's comet, others have been divided and subdivided 
so as finally to be separated into small fragments moving 
in the orbit of the original comet and thus constituting 
a meteoric ring or stream. 

234. The following general conclusions with regard 
to meteors in the solar system have been suggested: 
1. Biela's comet in 1845 passed very near, if not through, 

232. From what point do the November meteors seem to proceed ? Why ? Why 
iB their velocity double that of the earth ? 233. What are meteors supposed to be ? 



METEORS OR SHOOTING STARS. lJfl 

the November stream, and was probably divided in this 
way ; 2. The rings of Saturn are dense meteoric streams, 
the principal or permanent division being due to the dis- 
turbing influence of the satellites ; 3. The asteroids are a 
stream or ring of meteors, the largest being the Minor 
Planets which have been discovered. 

234. What general conclusions have been suggested ? 



SECTION IX. 



THE STAES. 



235. The Stars are luminous bodies like the sun, 
but situated at so vast a distance from the earth that 
they seem like brilliant points, and always in nearly the 
same positions with respect to each other. They are 
readily distinguishable from most of the planets by their 
scintillation or twinkling, which is caused by the irregu- 
larities in density, moisture, etc., of the different strata 
of the atmosphere through which the rays of light pass. 

236. The distance of the nearest star is found by cal- 
culation to be more than 20 trillions of miles, — a dis- 
tance so vast that light, traveling at the rate of 184,0(11 
miles a second, requires more than 3 J years to reach us 
from that remote luminary. Other stars are known to 
be more than twenty times as distant. 

237. The stars are divided into classes according to their 
apparent brightness, the brightest being distinguished as 
stars of the first magnitude, the next of the second, and 
so on. Stars of the first six magnitudes are visible to 
the naked eye ; but the telescope reveals the existence 
of others so feeble in light as to be classed as of the 
seventeenth magnitude. 



235. What are the stars ? How may they he distinguished from the planets ! 
What causes the scintillation ? 236. What is the distance of the nearest star ? 
How distant are other stars known to be ? 237. How are the stars divided ? Which 
are visible to the naked eye ? Remark ? 



THE STARS. 1J/3 

This classification is based merely on appearance, and therefore 
indicates nothing as to the real magnitudes of the bodies in ques- 
tion. The average brightness of stars of the first magnitude is one 
hundred times as great as that of stars of the sixth ; but Sirius, 
the brightest star in the heavens, is more than three times as bright 
as an average star of the first magnitude. 

238. The whole number of stars visible to the naked 
eye appears to be very great, — almost countless. There 
are, however, in the northern hemisphere only 2,400 ; 
and in both hemispheres, about 4,500. Viewed through 
the telescope, these numbers swell into millions. 

239. The Constellations.— To facilitate the naming 
and location of the stars, the heavens are divided into 
particular spaces, represented on the globe or map as 
occupied by the figures of animals or other objects. 
These spaces and the groups of stars which they contain 
are called Constellations. Thus there are the constella- 
tions Aries, the Ram ; Leo, the Lion ; Gemini, the 
Twins, etc. 

240. The general position of a star is denned by 
stating in what part of the figure it is situated ; as, the 
eye of the Bull, the heart of the Lion, etc. Its exact 
position is, of course, only to be defined by its right as- 
cension and declination, or longitude and latitude. This 
system of grouping the stars into constellations is sup- 
posed to be very ancient. Ptolemy counted only forty- 
eight constellations ; but, since his time, the number has 
been augmented to 109. 

233. Number of the visible stars ? 239. What are constellations ? Give examples. 
240. How is the position of a star defined ? Antiquity of the system ? 



m 



THE STARS. 



241. The most conspicuous stars of each constellation 
have particular names, as SirHus, Arctu'rus, Reifulum 
etc. ; but the most general designations are the letters 
of the Greek alphabet, a (alpha) being given to the 
brightest star, 13 (beta) to the next, and so on. "When 
the twenty-four letters of this alphabet are exhausted, the 
Roman letters are used, and subsequently the Arabic 
numerals, the latter being applied according to the posi- 
tions of the stars in the constellation, the most eastern 
being designated 1, which is thus the first star to cross 
the meridian. 

The Greek Alphabet. — The following are the letters of rhe 
Greek alphabet, with their names. It will be convenient for the 
student to become familiar with them, as they are very frequently 
employed. 



a Alpha 

|8 Beta 

7 Gamma 

6 Delta 

e Epsilon 

; Zeta 



V Eta 

d Theta 

i Iota 

k Kappa 

a Lambda 

H Mu 



v Ku 

£ Xi 

o Omicron 

7T Pi 

p Rho 

a Sigma 



r Tau 

v Upsilon 

$ Phi 

X Chi 

V Psi 

u Omega 



In the literal designations the letter is followed by the Latin 
name of the constellation, in the genitive or possessive case. Thus, 
a Gentauri means the brightest star of Centaurus ; (3 Tauri, the sec- 
ond star of Taurus ; y Andromeda, the third star of Andromeda, etc. 

242. The constellations are distinguished as Northern, 
Zodiacal, and Southern, according to their positions in 
the heavens with respect to the ecliptic. The zodiacal 



241. What stars have particular names ? What is the geucral mode of design* 
tion ? 242. How are the constellations distinguished ? What is the situation of 
the zodiacal constellations ? Why do they not correspond to the signs ? 



THE STARS. 1^5 

constellations have the same names as the signs, but are 
situated about 28° to the east of them, so that Aries, 
although the first sign of the ecliptic, is the second con- 
stellation of the zodiac. This is caused by the Precession 
of the Equinoxes. 

243. Precession. — By precession is meant the grad- 
ual falling back of the equinoctial points from east to 
west. In other words, the sun, in his apparent annual 
revolution around the earth does not cross the equinoc- 
tial always at the same points, but at every revolution 
crosses a little to the west of where it crossed previously. 
The amount of precession is about 50 seconds every year, 
an entire revolution of the equinoxes requiring a period 
of nearly 26,000 years. 

244. This movement of the equinoxes is the result of 
the spheroidal form of the earth. For since the excess 
of matter at the equator is situated out of the plane of 
the ecliptic, the attraction of the sun and moon acts ob- 
liquely upon it, and thus tends to draw the planes of the 
equinoctial and ecliptic together; which tendency, by 



j v S 




243. What is precession ? What is its amount ? 244. How is it caused ? Explain 
by the diagram. 

19 



lJfi THE S TA R S. 

the rotation of the earth on its axis, is converted into a 
sliding movement, as it were, of one circle upon the 
other, both preserving very nearly the same inclina- 
tion. 

This is illustrated in the annexed diagram. The attraction of 
the sun acting obliquely upon the protuberance, or excess of 
matter, at E and E', tends to draw it toward the plane of the eclip- 
tic; and this it would finally accomplish were the earth's rotation 
suspended ; so that the plane of the equator would be made to 
coincide with that of the ecliptic. But the effect is a sliding of 
the equator over the line of the ecliptic, and thus a change of the 
points of intersection. 

245. Since the equator moves round on the ecliptic, 
the poles of the earth must revolve around those of the 
ecliptic, and consequently change their apparent position 
among the stars. Hence, the star which is now so near 
the north celestial pole will not always he the pole-star; 
hut in about 13,000 years, that is, one-half the period of 
an entire revolution, will be 47° from it. The apparent 
places of all the stars undergo a constant change, for 
the same reason, — their right ascensions and declina- 
tions increasing or diminishing according to their situa- 
tion. 

246. The following are the names of the principal 

constellations and of the brightest stars in each. The 
student should be familiarized with their situations by 
pointing them out on the globe or on a planisphere. 

245. What change occurs in The position of the pole-star ? In the apparent places 
of the stars ? 246. What are the names of the principal Northern Constellations \ 
Of the Zodiacal Constellations ? Of the Southern Constellations ? Name the prin- 
cipal stars in each. Name the constellations that are on the meridian nearly at the 
same time as each of the Zodiacal Constellations. 



THE STARS. 1^7 



NORTHERN CONSTELLATIONS. 

Na^te. Meaning. Brightest Stars. 

Andromeda The Chained Princess. . Alpneratz, Mirach, 

AJmaack. 

Aq'ctla The Eagle Altair. 

Auriga The Charioteer Capel'la. 

Bo-o'tes The Bear Hunter Arctu'rus. 

Ca'nes Venat'ici The Hunting Dogs. 

Cassiopeia The Queen in her Chair. Sche'dar, Capn. 

Ce pheus The King Alderamin. 

Cltp'eus Sobies Kn . , . . Sobieski's Shield. 

Coma Berenices Berenice's Hair. 

Corona Borea lis The Northern Crown. . . Alphac'ca. 

Cyg'nus The Swan Arided, Albir'eo. 

Delphi'nus The Dolphin Svalocin. 

Draco The Dragon Ras'taben. 

Her cules Hercules Ras Algethi. 

Leo Mlnor. The Lesser Lion. 

Lyra The Harp Vega. 

Peg'asus The Winged Horse Markab, Sclieat. 

Perseus et Caput { Perseus and Medusa's Al genib, 

Medu'SuE } Head Algol. 

Sagit'ta Hie Arrow. 

Serpens The Serpent. 

Taurus Vojsiatow' SKii.Poniatowski's Bull. 

Triangulum The Triangle. 

„ ,, m , A lT , ( Dublie, Merak, Al 

Ursa Major The Great Bear \ . ' _ _. 

( 10th, Mizar. 

Ursa Mlnor Tlie Lesser Bear Polaris. 

Yulpec'ula et Anser, . The Fox and the Goose. 

ZODIACAL COMBINATIONS. 

Aries The Ram Hamal {a Arietis). 

Taurus The Bull Aldeb'aran, Alcyone. 

Gemlni The Twins Castor, Pollux. 

Cancer The Crab.. 



lJfS THE STARS. 

Name. Meaning. Bright Stars. 

Leo The Lion Reg'ulus, Deneb'ola. 

Virgo The Virgin Spica, Vindem'iatrix. 

Libra The Balance Zuben el Genubi, Zu- 

ben el Chamali. 

Scorpio The Scorpion Antares. 

Sagittarius The Archer. 

Capricornus The Goat. 

Aquarius The Water-bearer. 

Pisces The Fishes. 



SOUTHERN CONSTELLATIONS. 

Argo Navis The Ship Argo. 

Canis Major The Great Bog Sir'ius. 

Canis Minor T7ie Lesser Dog Procy'on. 

Centaurus The Centaur. 

Cetus The Whale Menkar, Mira. 

Corona Australis The Southern Crown. 

Coryus The Crow Alchiba, Algorab. 

Crater The Cup. 

Crux The Cross. 

Erid'anus The River Po Acher'nar. 

Hydra ... The Water Serpent Alphard, or Cor Hy- 
dra?. 

Lepus The Hare Arneb. 

Lupus The Wolf. 

Monoc'eros The Unicorn. 

Ophiu'cus The Serpent Bearer Ras Alhagus. 

Orion The Hunter Betelgeuse, Rigol, 

Bellatrix. 

Phcenix The Phoenix. 

Pisces Australis Tlie Southern Fish Fom'alkaut 



THE STABS, 



149 



TABLE SHOWING THE POSITIONS OF THE CONSTELLA- 
TIONS IN THE HEAVENS. 

Note. — Each line in this table represents about 30° of right 
ascension. 



Xokth Declination. 



South Declination. 



90°-50° 


| 50°-25° 


25 c — 0° 1 Zodiac. 


0°— 25° 


2o°— 50° 


Cassiopeia 


Andromeda 
Triangulum 

Perseus 




fisces 
Aries 

Taurus 


Cetus 

Lepus ( 
Orion j 


Phoenix 

Eridanus 

Columba 


Ursa Major 


Auriga 
Leo Minor 


Canis Minor 


Gemini 
Cancer 

Leo \ 


Canis Major 
Monoceros 

Hydra 
Crater 


Argo 




Canes Yen. 


Coma Ber. 1 Virgo 


Connas 


Centaurus 


Ursa Minor ■ 
Draco 


Bootes 
Corona Bor. 
Hercules 


Serpens .Libra 
Taurus Pon. Scorpio 


Ophiuehus 


Lupus 




Lyra -j 


lq?na a Sagittarius 


Clyp. Sobiesk. 


Corona Aus. 


Cepheus 


Cygnus 


Delphinus 
Pegasus 


Cnpricornus 

Aquarius 







Each column of this table contains the constellations as they 
are arranged from west to east ; and each line read from left to 
right, gives the constellations, from north to south, that are on or 
near the meridian at the same time. By knowing the time that 
each zodiacal constellation comes to the meridian, remembering 
that these constellations are about 30° east of the signs, and that 
those are on the meridian at midnight which are opposite to the 
sun's place at noon, the student with a little consideration will be 
able to find the position of the constellations at any hour and on 
any evening during the year. The position at any time of the 
evening can easily be deduced from that at midnight by reckoning 
for each hour 15°, toward the east if the time is earlier, and toward 
the west, if later. 



150 TEE STARS. 

The following is a list of the most noted or conspicu- 
ous of the stars, with their names and situations : 

LIST OF PRINCIPAL STARS. 

Name. Situation. R A. Dec. 

Sirius Nose of the Great Dog 100° 16^° S. 

Canopus The Ship Argo 95° 52^° S. 

Arcturus Knee of Bootes 212° 20° N. 

Betelgetjse Shoulder of Orion 87° 7£° N. 

Rigel Foot of Orion 77 c 8^° S. 

Capella Goat of Auriga 77° 46° N. 

Vega One of the strings of the Harp. . . 278° 39 ' N. 

Procyon.... The Little Dog 113°.... 5^ c N. 

Achernar The River Po 23° 58° S. 

Aldebaran Eye of the Bull 67° ... . 16i° N. 

Antares Heart of the Scorpion 245° 26° S. 

Altair Neck of the Eagle 300° 8.|° N. 

Spica Sheaf of Virgo 200° 10|° S. 

Fomalhaut Southern Fish 343° 30|° S. 

Regulus Heart of the Lion 150° \%\° N. 

Deneb Tail of the Swan 309° 45° N. 

Alpheratz Head of Andromeda £° 28^° N. 

Dubhe Great Bear 164° 62£° N. 

Castor ) TT _' _ .. (112° 32° N. 

_ - Heads of Gemmi ^, nol0 . T 

Pollux S \ 114° 28i N. 

Pole-Star Tail of the Little Bear 18£° 88|° N. 

Alphard Heart of Hydra 140° .... 8° S. 

Ras Alhagus Head of the Serpent-bearer 262° 12£° N. 

Markab Wing of Pegasus 345° 14i N. 

Scheat f. Thigh of Pegasus 345° 27£° N. 

Algenib Wing of Pegasus 2° 14£° N. 

Algol Head of Medusa 45° 40^° N. 

Denebola Tail of the Lion 176° 15° N. 

Alphecca Northern Crown 232°. . . . 27 : N. 

Benetnasch Tip of the Great Bear's Tail 206° 50° N. 

Alderamin Breast of Cepheus . . 318° .... 62° N. 

Vindemiatrix Right Arm of Virgo 194° \\\° N. 

COR Caroli The Hunting Dogs 193° 39° N. 

Alcyone The Pleiades 55 c 23f N. 



THE STABS. 151 

PROBLEMS FOR THE CELESTIAL GLOBE. 

PROBLEM I. 

To find the place of a constellation or star on the globe : 
Bring the degree of right ascension belonging to the con- 
stellation or star to the meridian ; and under the proper 
degree of declination will be the constellation or star, 
the place of which is required. 

jSTote. — The student should be exercised in finding the places of 
all the principal stars laid down in the list according to this rule. 
The place of a planet or comet may also be found by this rule 
when its right ascension and declination are given. 

PROBLEM II. 

To find the appearance of the heavens at any place, 
the hoar of the day and the day of the month being given : 
Make the elevation of the pole equal to the latitude of 
the place ; find the sun's place in the ecliptic, bring it to 
the meridian, and set the index to 12. If the time be 
before noon, turn the globe eastward ; if after noon, turn 
it westward till the index has passed over as many hours 
as the time wants of noon, or is past noon. The surface 
of the globe above the wooden horizon will then show 
the appearance of the heavens for the time 

Note. — The student must conceive himself situated at the centre 
of the globe looking out. 

PROBLEM III. 
To find the declination and right ascension of any 
constellation or star : Proceed in the same manner as to 
find the latitude and longitude of a place on the terres- 
trial globe. 



152 THE STARS. 

247. The Galaxy, or Milky Way, is that faint lu- 
minous zone which encompasses the heavens, and which, 
when examined with a telescope, is found to consist of 
myriads of stars. Its general course is inclined to the 
equinoctial at an angle of 63°, and intersects it at about 
105° and 285° of right ascension. Its inclination to the 
plane of the ecliptic is consequently about 40°. Its ap- 
pearance is not uniform, some parts being exceedingly 
brilliant ; while others present the appearance of dark 
patches, or regions comparatively destitute of stars. 

The number of stars in the Milky Way is inconceivably great. 
Sir William Herschel states that on one occasion he calculated that 
116,000 stars passed through the field of his telescope in a quarter 
of an hour, and on another that as many as 258,000 stars were seen 
to pass in 41 minutes. The total number, therefore, can only be 
estimated in millions. 

248. The prevailing theory with regard to the Milky 
Way is, that it is an immense cluster of stars having the 
general form of a mill-stone, split at one side into two 
folds, or thicknesses, inclined at a small angle to each 
other ; that all the stars visible to us belong to this sys- 
tem ; and that the sun is a member of it and is situated 
not far from the middle of its thickness, and near the 
point of its separation. 

The fact that the Milky Way is composed of vast numbers of 
stars was conjectured by Pythagoras and other ancient astrono- 
mers, but was not positively discovered till Galileo directed his 
telescope to the heavens. The hypothesis that it is a vast cluster 



247. WTiat is the galaxy or milky way ? Its general course ? Its appearance ? 
The number of stars that compose it ? 248. What is the prevailing theory in regard 
to it ? History of the hypothesis ? 



THE STARS. 153 

of which the sun and visible stars are members was first suggested 
by Thomas Wright in a work entitled the " Theory of the Uni- 
verse," published in 1750. This subject received a careful and 
prolonged investigation by Sir William Herschel, the results of 
which he published in 1784, and which seems to establish the 
hypothesis mentioned in the text. This opinion he arrived at by 
taking observations at different distances from the zone of the 
Galaxy, and counting the stars within the field of view. On the 
supposition that the stars are uniformly distributed throughout 
the system, the number thus presented would indicate the extent 
of the cluster in the direction in which they were seen; and in this 
manner some general idea of its form would be obtained. 




SECTION OP THE GALACTIC STRATUM, 



The annexed figure represents the general form of a section of 
this vast cluster, S being the position of the sun. 

249. Proper Motion of the Stars. — The stars do 
not always remain precisely in the same places with re- 
spect to each other, but in long periods of time percep- 
tibly change their relative positions, some approaching 
each other, and others receding. This apparent change 
of position is called their proper motion. 

349. What is meant by the proper motion of the stars ? 

20 



154 



THE STARS. 



250. Herschel, finding that in one part of the heavens 
the stars are approaching each other, while in the oppo- 
site part their relative distances are increasing, arrived at 
the conclusion that the change in the position of the 
stars is caused by a motion of the solar system in space. 
For, evidently, if we are in motion, the stars toward 
which we are moving will open out, while those from 
which we are receding will appear to come together. 
Careful observations of this kind indicate that the system 
is moving toward a point in the constellation Hercules; 
and it is estimated that the velocity of the motion is 
about 160 millions of miles in a year. The central sun 
has been thought by some to be Alcyone, the principal 
star of the Pleiades. 



251. Multiple Stars are those which to the naked 
eye appear single, but when viewed through a telescope 
are separated into two or more stars. Those that con- 
sist of two stars are 
called double stars. 
They differ very 
greatly in their dis- 
tance from each oth- 
er, their separation 
in some cases re- 
quiring the most powerful telescopes, while in others the 
slightest optical aid is sufficient. The members of 
double stars are generally quite unequal in size, and 




1. POLE-STAR ; 2. RIGEL ; 3. CASTOR ; 4. y VIRGINIS. 



250. What conclusion did Herschel arrive at ? What does observation indicate ? 
What star is supposed to be the central sun? 251. What are multiple stars? 
Double stars ? How do they differ ? 



THE STARS. 155 

very often exhibit the beautiful phenomenon of different 
colors. 

252, Binary Stars are double stars one of which re- 
volves around the other, or both revolve around their 
common centre of gravity. 

The discovery of this connection between the constituents of 
double stars was, perhaps, the grandest of Sir William Herschel's 
achievements. It was announced by him in 1803, after twenty-five 
years of patient observation, which he commenced with a view to 
discover the stellar parallax by noticing whether any annual change 
in the relative positions of double stars existed. To his astonish- 
ment, he found from year to year a regular progressive movement 
of some of these bodies, indicating that they actually revolve one 
round the other in regular orbits, and thus that the law of gravita- 
tion extends to the stars. These stars are called Binary Stars, or 
Systems, to distinguish them from other double stars which, 
although perhaps at immense distances from each other, appear in 
close proximity, because, as viewed from the earth, they are very 
nearly in the same visual line, and therefore are said to be optically 
double. 

253. The observations of Herschel resulted in the dis- 
covery of about 50 binary stars ; but since his time the 
number has been very greatly increased. Most of the 
double stars are believed to be binary systems. A very 
careful scrutiny of these bodies and their changes in posi- 
tion lias shown that they revolve in elliptical orbits of 
considerable eccentricity and in periods greatly varying 
in length. 



252. What are binary stars ? How discovered ? 253. Their number ? Their 
orbits ? 



156 THE S TA R S. 

254. Stars that appear double when viewed through a 
glass of low power are often separated by one of higher 
power into triple, quadruple, or other multiple stars. 

Thus, e Lyras, with very slight 
optical assistance, is resolved into 
two stars, each of which is a 
close double star; and 6 Orionis 
(Theta of Orion) consists of four 
bright stars, two of which have 
companion stars, thus forming a 
sextuple star. From the config- 
uration of the four principal 
stars, this is sometimes called 
the Trapezium of Orion. 




6 OBIONIS. TRAPEZIUM OF ORION. 



255. Variable Stars are those which exhibit periodi- 
cal changes of brightness. The number of such stars 
discovered up to the present time (1S70) is about 120. 
They are sometimes called Periodic Stars. 

One of the most remarkable of these stars, and the first noticed 
(by Fabricius in 159G), is Mira — the wonderful — in the Whale. It 
appears about 12 times in 11 years ; remains at its greatest bright- 
ness about a fortnight, being equal to a star of the 2d magnitude ; 
decreases for about 3 months, and then becomes invisible, remain- 
ing so 5 months, after which it recovers its brilliancy ; the period 
of all its changes being about 331 * days. 

Algol, in the Head of Medusa, is another remarkable variable star 
of a very short period, it being only 2d. 20h. 49m. It is commonly of 
the 2d magnitude, from which it descends to the 4th magnitude in 
about 3| hours, and so remains about 20 minutes, after which, in 



254. Into what are some double stars separated by the telescope ? Give examples. 
■255. What are variable stars ? Their number ? What are they sometimes called ? 
What examples are given ? 



THE STARS. 157 

ti\ hours, it returns to the 2d magnitude, and so continues 2d. 13h., 
when similar changes recur. 

255. Cause of Variable Stars. — Several hypotheses 
have been suggested to account for these interesting phe- 
nomena. One is that these bodies rotate, and thus pre- 
sent sides differing in brightness, or obscured by spots 
similar to those which are seen on the solar disk ; an- 
other, that their light is obscured by planets revolving 
around them ; and a third, that their light is diminished 
by the interposition of nebulous masses, since it has been 
observed that during their minimum brightness they are 
often surrounded by a Mud of cloud or mist. Xo one of 
these hypotheses is entirely satisfactory, and hence we 
must conclude that the true cause of the variability of 
these stars is unknown. 

257. Temporary Stars are those which suddenly 
make their appearance in the heavens, sometimes shining 
with very great brilliancy ; and, after a while, gradually 
fade away, either entirely disappearing or remaining as 
faint telescopic stars. The latter are properly called 
New Stars. 

Several instances are on record in ancient times. The star of 
1572 was a very remarkable one. It appeared first as a star of the 
first magnitude, blazing forth with the lustre of Venus, and visible 
even at noon. It lasted from November, 1572, to March, 1574. In 
1604, a very splendid star shone forth in the constellation Ophiu- 
chus, and lasted 15 months. Another in the same constellation 
appeared in 1848, which still remains as a telescopic star. Lastly, 
a new star was seen in May, 1866, in Corona Borealis. It first 

253. What is the cause of variable stars ? 257. What are temporary stars ? Men- 
tion some instances. ^. 



168 THE STARS. 

appeared of the second magnitude, and of a pure white color ; 
but in a week had changed to the fourth, and soon alter to the 
ninth. 

258. Cause of Temporary Star.3. — Xo satisfactory 
hypothesis has as yet been advanced to account for these 
phenomena. Some have supposed that these stars are 
revolving in elliptical orbits of great eccentricity so that 
they sometimes approach very near us and then recede 
to great distances ; but this is rendered improbable by 
the sudden changes in brilliancy ; since, to pass from the 
first to the second magnitude, it has been computed 
would require six years, if the star moved with the 
velocity of light ; whereas, that of 1572 underwent this 
change in one month, and that of 1S66 diminished to the 
extent of live magnitudes in the same time. Another 
hypothesis is, that extensive conflagrations take place 
on the surface of these bodies, which in their progress 
give rise to the observed changes in color aud bright- 
ness, and at their extinction leave the body in an obscure 
state. 

259. Numerous instances are on record of stars for- 
merly known to exist which have entirely disappeared 
from the heavens. These are called Lost or 2I'tssing 
Stars. 

Some of the instances mentioned by early astronomers, 
of lost stars, may be the result of erroneous entries ; but 
those of later times cannot possibly be accounted for in 
this way. Revolving in orbits, they may have passed 

258. What is the supposed cause of them ? 259. What are lost or missing stars ? 
now accounted for ? 



r 



THE STARS. 



159 



beyond the reach of the most powerful telescope ; or 
they be obscured by the interposition of great nebulous 
masses, and thus are only concealed for a certain period, 
which however may comprise hundreds, or even thou- 
sands of years. 

260. Star Clusters.— These are dense masses of stars 
so crowded together, and so far distant, that they pre- 
sent a hazy, cloud- 
like appearance, 
similar to that of 
the Milky Way. 
Collections of stars 
visible as such to 
the naked eye, al- 
though considera- 
bly crowded, are 
called Star-groups. 
Such are the Ple- 
iades, the Ilf/ades, 
and the group 
which constitutes 
the constellation 
Berenice's Hair. 

Among star-clusters, a very small number are suffi- 
ciently bright to be distinguished by the naked eye ; 
but generally they require a telescope to render them 
visible. 

The annexed cut represents a peculiarly magnificent object of 
this kind, situated between Eta and Zeta in the constellation Her- 




cixster in hercui.es.— Sir J. Herschel. 



260. What are star clusters ? Star groups ? Example ? 



160 TEE STARS. 

cules. On a very clear night, it is visible to the naked eye as a 
small nebulous spot or faint star. In the telescope its appearance 
is considerably changed by several outlying branches, while its 
condensation at the central portions is quite a striking feature. 
Very many objects of a similar character are visible in different 
parts of the heavens. 

261. The number cf stars contained in these clusters 
is very great. According to Arago, many clusters con- 
tain at least 20,000 collected in a space the apparent 
dimensions of which are scarcely a tenth as large as 
the disk of the moon. The clusters are not equally dis- 
tributed over the heavens, but are most numerous in the 
Milky Way; while globular clusters most abound in 
that region of the Galaxy which is contained between 
Lupus and Sagittarius in the southern hemisphere. 

These globular clusters are supposed to be held together by their 
motions and mutual attractions. That there must be a real con- 
densation is obvious from a simple glance at such an object as 
that depicted in the figure on page 159; since the increase of 
brightness toward the centre is far too great to be explained on 
the supposition that the stars are equally distributed, but appear 
closer together at the centre, because the visual line traverses there 
a much greater portion of the mass. 

261. What is the number of stars contained in these clusters ? Their distribution ? 



SECTION X. 



XEBULil. 



262. Nebulas are certain faintly luminous appear- 
ances in the heavens, resembling specks of cloud or mist, 
some just visible to the naked eve, but the greater part 
only to be discerned with a telescope. They resemble 
in their general apects the distant star-clusters, but their 
physical structure appears to be very different. 

Their distance from us must be immense, since they 
constantly maintain very nearly the same situation with 
respect to each other and to the stars. Their magnitudes 
also must be inconceivably vast. 

The first of these objects mentioned in the annals of astronomy 
was discovered in 1612, by Simon Marius, a German astronomer. 
This was the nebula situated in the girdle of Andromeda. In 1650,. 
Huyghens discovered the great nebula in Orion. The labors of Sir 
William Herschel, directed to the investigation of this department 
of astronomy for more than twenty years, enabled him in 1802 to 
publish a catalogue of 2,500 nebula and clusters ; and the subse- 
quent researches of his son, Sir John Herschel, in the southern 
hemisphere, has increased this number to more than 5,000. Very 
great additions, however, have been made to our knowledge fif 
these interesting objects by the labors of Lord Rosse, aided by the 
largest reflecting telescope ever constructed. 

362. What are nebulte ? Their distance from us ? Example ? 

21 



1G2 NEBULAE. 

263. Nebulas are distinguished from clusters by not 
being resolved into stars when viewed through the most 
powerful telescopes, presenting the appearance of diffuse 
luminous substances, filling vast regions of space, and 
differing in form, and degree of condensation. 

Herschel at first thought all nebula; resolvable into star- ; 1-ut 
his subsequent investigations convinced him that this was an error ; 
and he accordingly divided these objects into resolvable and irresolv- 
able nebula; the first being those vast star-clusters which exhibit a 
nebulous, or cloudy aspect, because of their comparatively crowded 
condition and great distance from us ; and the second, according 
to his conceptions, immense aggregations of self-luminous matter, of 
great tenuity, but gradually condensing into solid bodies like the 
sun and stars. 

alany of the irresolvable nebula? of Sir William Herschel having 
been resolved by the great telescope of Lord Rosse, or having given 
indications of being resolvable into stars, the opinion came to be 
almost universally entertained that all nebula are star-clusters, some 
so distant that light requires millions of years to pass from them to 
us. But the more recent researches by means of the analysis of 
light, have proved that these luminous masses consist of gaseous 
not solid matter ; so that Herschel's hypothesis would seem to be 
established. These diffuse and attenuated substances constitute 
thus a peculiar class of objects in the starry heavens, and are the 
nebula defined in the text, although some astronomers still continue 
to classify them with the clusters which have a nebulous appear- 
ance. 

264. Nebulae may be divided, according to their form, 
into the following six classes : Elliptic, Annular, Sp-irdtl 

Planetary, Stellar, and Irregular Nebula*. 



263. How are they distinguished from clusters ? nerschel's hypotheses ! What 
has heen proved by recent re-earche3 ? 954. How may nebula.' be divided? 



NEB UL^E. 



163 



265. Elliptic Nebulae are such as 

have the elliptical or oval form. They 
are quite numerous, and of various 
degrees of eccentricity. The one al- 
ready referred to in Andromeda is an 
example. There is another in the 
same constellation, near the star 
Gamma, which is represented in the 
annexed figure. 

266. Annular Nebulae arc such as 
have the form of a ring. These are 
very rare, the heavens affording only 
four examples. 

The most remarkable one is found in Lyra, situated between the 
stars Beta and Gamma, and may be seen with a telescope of mod- 
erate power. It is slightly elliptical and has the appearance of a 




ELLIPTIC NEBULA IN AT- 
DKCCffEDA. 




annular nebula in LTKA.— 1. Sir John Herschel ; 2. Lord Bocse. 

flat oval ring, the opening occupying somewhat more than one-half 
of the diameter. The central portion, when viewed through a 
powerful telescope, is not altogether dark, but is crossed with faint 



265. What are elliptic nebulae ? Examples ? 266. What are annular nebula: ? 
Examples ? 



lGlf. NEBULJE. 

nebulous streaks, compared by some to gauze stretched over a hoop. 
The telescope of Lord Rosse shows fringes of stars at its inner and 
outer edges. The other annular nebula; are two in Scorpio, and 
one in Cygnus. 

267. Spiral Nebulas are such as have the form of one 
or more spirals or coils ; in some cases presenting the 
appearance of continuous convolutions, or whorls ; in 
others, of great spiral arms or branches projecting from 
a central nucleus. 




SPIRAL NEBUL.E IN LEO.— LOI'd RoSSe. 

The discovery of nebulae of this remarkable form is due to Lord 
Rosse, no indication of it whatever having been afforded by the 
great telescope of Sir William Herschel. The grandest object of 
this kind is found in Canes Venatici. Brilliant spirals, unequal in 
sizs and brightness, and apparently overspread with a multitude 
of stars, diverge from the central nucleus, the whole suggesting the 
idea of a rotary movement of considerable rnpidity, and the play 
of forces at which the imagination is startled when it contemplates 
the immensity of space filled by this wondrous object. 

268. Planetary Nebuleo arc those which, in their cir- 

237. What are spiral nebulae? By whom discovered? Example? 2G3. What 
are planetary nebula? ? Examples ? 



NEBULAE. 165 

cular or slightly elliptical form, their pale and uniform 
light, and their definite outline, resemble the larger and 
more distant planets of our system. 




PLANETARY NEBULA. 1, IN* TRSA MAJOR ; 2, IN ANDROMEDA. 

One of the most striking of this class is found in Ursa Major 
(near ,3), the light of which, in Sir John Herschel's drawing, is 
quite uniform ; but when seen through Lord Rosse's telescope, it 
presents the appearance depicted in the annexed cut (ISTo. 1). The 
disk is about 8' in diameter, and exhibits a double luminous circle 
with two dark openings, each containing a bright but partially 
nebulous star. No. 2 in the same figure, represents a nebula near 
k (Kaypa) in Andromeda, which, though perfectly round in Her- 
schel's drawing, appears in Lord Rosse's like a luminous ring sur- 
rounded by a wide nebulous border. In the illustration on page 166, 
1 and 2 are representations of planetary nebulas. 

269. Stellar Nebulae are those which appear to en- 
velop one or more brilliant spots or points, resembling 
stars surrounded by a nebulous border or ring. Some 
of these are called nebulous stars. If the nebula is cir- 



269. What are stellar nebulas ? What are some of them called ? Where is the 
star situated ? Examples ? 



XED UL JB. 



cular, the star occupies the centre of it ; but some that 
are elliptical have two stars situated at the foci of the 
ellipse. 




In the above cut, No. 5 represents a remarkable nebulous star in 
Oygnus. The star is of the 11th magnitude, and is at the centre of 
a perfectly circular nebula of uniform light, and about 15' in diam- 
eter. No. 4 is a stellar nebula in Sobieski's Shield, of an elliptic 
form, and having two stars at the foci of the ellipse. These stars 
are described by Sir John Herschel as of a gray color. No. 3 is 
the representation of a nebula bearing a resemblance to a comet. 
It is found in the tail of Scorpio. There are several other instances 
of such nebulae, which from their appearance are called conical or 
cometary nebulce. In the case of each, the stellar or bright point is 
at one extremity of the nebulous mass. 

270. Irregular Nebulae are such as have no symme- 
try of form and scarcely any distinctness of outline, and 
r.r3 also remarkable for the diversity of brightness which 
they exhibit at different parts. 

Arago lemarks of these diffuse masses of nebulous matter, that 
k - they present all the fantastic figures which characterize clouds 
carried away and tossed about by violent and often contrary 
winds." The most remarkable of these objects are the following : 

1. The Crab Nebula in Taurus. — This singular object has an ellip- 
tic outline in ordinary telescopes, but in Lord Rosse's great reflector 
it presents an appearance which has been fancifully likened to a 
crab or lobster with long claws. 

270. What are irregular nebulos ? What examples are given ? 



NEB UL^E. 



167 



2. Tiie Great Nebula in Orion. — This is probably the most inag- 
lificent of all the nebulse. It is very irregular in form ; of immense 
ixtent, covering a surface more than 40' square ; and consists of 
>atches varying considerably in brightness. Near the famous sex- 




CKAB NEBULA IN TAUBTTS.— Lord EoSSe. 

;uple star d Ononis, already described, it is very brilliant ; but 
)ther portions are quite dim, and some absolutely black. It was 
•nought that portions of this nebula had been resolved into stars 
3y the telescopes of Lord Rosse and Prof. Bond ; but recent obser- 
vations have proved conclusively the gaseous nature of this object. 
8. The Great Nebula i/i Argo. — This is another very irregular and 
?xtensive nebula, covering a space equal to five times the disk of 
;.he moon. It contains a singular vacancy of an irregular oval 
form near the centre, and not very far from the variable star Eta. 
'It is not easy," says Sir J. Herschel, "to convey a full impression 



168 



XE D ULjE. 




dumb-bell nebula. — Hersche!. 



of the beauty and sublimity of the spectacle which this nebula 
offers as it enters the field of the telescope, ushered in as it is by so 
glorious a procession of stars, to which it forms a sort of climax.'' 
This nebula is remarkably destitute of any indications of resolva- 
bility. 

4. The Dumb-bell Nebula.— Thi4 
object is found in Vulpecula, and 
derives its name from its singular 
appearance as viewed through a 
telescope of moderate power. In 
Lord Rosse's telescope it assume* a 
form of less regularity, and appears 
to consist of innumerable stall 
mixed with a mass of nebulous 
matter. These may be only centre:; 
of condensation. 
5. The Magellanic Clouds. — These are situated in the southern 
hemisphere and not far from the pole, auel are called sometimes 
Nubecula Major and Minor, or the Greater and Lesser Cloudlets. The 
former is in Dorado ; the latter in Toucan. These objects arc dis- 
tinguished for their great extent, the larger one covering a space 
of about 42 square degrees, and the smaller being of about or.c- 
fourth that extent, but of greater brightness. 

271. Double Neb- 
ulae are those which 
indicate by their clod 

proximity to each other 
that they have a physi- 
cal connection. Mori 
than 50 of such objects 
have been discovered, 
the component nebuHj 
of which are not more 
than 5' apart. 




double nebula.— Lord Souse. 



NEBVLjE. 1u9 

The annexed cut represents an object of this kind, found in 
Gemini. It is composed of two rounded masses, terminated by 
brilliant appendages and enveloped in a nebulous mass, the whole 
surrounded by light luminous arcs resembling fragments of a 
nebulous ring. 

272. Variable Nebulae are those which undergo 
changes in apparent form and brightness. 

Several instances of such changes have been positively ascer- 
tained by Struve, D'Arrest, Hind, and other distinguished astrono- 
mers. The great nebula in Orion and Argo have exhibited un- 
doubted variations of a marked character. 

273. Structure of the Universe. — The universe has 
been supposed, by many modern astronomers, to consist 
of an infinite number of star-clusters similar to the 
galaxy, and situated at inconceivably immense distances 
from it and from each other. In view, however, of the 
recent discoveries as to the nature of the nebulas proper, 
this hypothesis cannot be considered as established ; and 
the true structure of the universe remains a problem to 
be solved. 



271. What are double nebulae ? Their number ? Describe the one in Gemini. 
272. What are variable nebula? ? What instances are mentioned ? 273. What is 
said of the structure of the universe ? 



22 



SECTION XI. 



274. The apparent motions of the sun and stars, 
caused by the real motions of the earth, afford standards 
for the measurement of time. The time which elapses 
between a star's leaving the meridian of a place until it 
returns to it again is called a sidereal day. 

This is the time of one complete revolution of the celestial sphere, 
and is the exact period of one rotation of the earth on its axis. It is 
an absolutely uniform standard, having undergone not the slightest 
appreciable change from the date of the earliest recorded observa- 
tions. Indeed, it is the only absolutely uniform motion observed 
in the heavens. 

275. A Solar Day is the period which elapses from 
the sun's leaving the meridian of a place until it returns 
to it again. 

As the sun is constantly changing its place among the stars, 
owing to the annual revolution of the earth, this period must be 
longer than a sidereal day ; for the sun having moved toward the 
east during the time of a rotation, the earth must turn farther in 
order to bringr the place again into the same relative position with 
the sun. This will be understood by examining the annexed 
diagram. 



274. What afford standards for measuring: timo? What is a sidereal day? 
°T5. What is a solar day? Why longer than a sidereal day? Explain from tho 
diagram. 



■■■ 



TIZIE. 171 



Let 1 represent the earth in one position of its orbit, and 2 the 
position to which it advances during one day ; P, the place at 




which the sun is on the meridian at 1 ; P', the same place after one 
complete rotation, as shown by the parallel F' S. It will be evi- 
dent that, in order to bring P' under the meridian, so that the sun 
may appear to cross it, the earth will have to turn a space repre- 
sented by the arc F' M, which will make the solar day so much 
longer than the sidereal day. 

276. The solar day exceeds the sidereal day by an 
average difference of four minutes ; but, owing to the 
variable motion of the earth in its orbit and the obliquity 
of the ecliptic, this difference is not the same throughout 
the year; and, consequently, the solar days are of un- 
equal length. 



276. How much longer are the solar days than the sidereal days ? Why are the 
solar days unequal ? 



172 



TIME, 



Why the Solar Days are Unequal. — That the variable motion 
of the earth in its orbit should produce this effect will be obvious 
from an inspection of the preceding diagram ; since it will be at 
once apparent that the length of the arc P' M must depend upon 
the length of the interval between 1 and 2. If these intervals vary, 
the arcs which represent the excess over a rotation turned by the 
earth in order to bring the sun on the meridian, must also vary, 
and in the same proportion. Hence, they must be longest when 
the earth is in perihelion, and shortest when it is in aphelion. 

The second cause, 
namely, the obliqui- 
ty of the ecliptic, 
may be explained 
by the annexed dia- 
gram : — Let API 
represent the north- 
ern hemisphere, AEI 
the equinoctial, and 
A e I the ecliptic. 
Let the ecliptic be divieled into equal portions, A b, h c, e <h etc., 
and elraw meridians through the points of division, intersecting the 
equinoctial in B, C, D, etc. The divisions of the ecliptic will be 
equal arcs of longitude, and the divisions of the equinoctial will be 
the corresponding arcs of right ascension, anel hence passed over 
by the sun in equal periods of time. These arcs of right ascension, 
it will be apparent, are not equal ; for A 5, which is oblique to 
A B, must subtend a smaller arc, A B, than d e which is nearly 
parallel to its arc D E. Thus the arcs of right ascension are short- 
est at the equinoxes, and longest at the solstices; while the divi- 
sions coincide at all these four points. 

277. A Mean Solar Day is the average of all the 
solar days throughout the year. It is divided into 
twenty-four hours, and commences when the sun is on 




277. What is a mean solar day ? A civil day ? 



TIME. 173 

the lower meridian, that is, at midnight. Because used 
for the general purposes of civil and social life, it is also 
called the civil day. Clocks are regulated to show its 
beginning and end, and the equal division of it into 
hours, minutes, and seconds. As already stated, it is four 
minutes longer than a sidereal day. 

278. The Equation of Time is the difference between 
apparent and mean time ; that is, the difference between 
time as shown by the sun, and that shown by a well- 
regulated clock. 

If the solar days were equal in length, the sun would always be 
on the meridian at 12 o'clock ; that is, apparent noon would coin- 
cide with mean noon — the noon of the clock. But this is not the 
case, and therefore to make the observed noon, as indicated by the 
sun, correspond with the noon of the clock, a correction has gene- 
rally to be made, either by adding or subtracting a certain amount 
of time. This is what is meant by the equation of time. 

The unequal motion of the earth in its orbit causes the sun to be 
in advance of the clock from aphelion to perihelion, that is, from 
July 1st to January 1st ; and behind it from January 1st to 
July 1st ; while they both coincide at those points. The obliquity 
of the ecliptic causes the sun to be in advance of the clock from 
Aries to Cancer, behind it from Cancer to Libra, in advance again 
from Libra to Capricorn, and behind again from Capricorn to Aries ; 
and makes them both agree at those four points. When these two 
causes act together, as is the case in the first three months and the 
last three months of the year, the equation of time is the greatest. 

279. The equation of time is greatest in the beginning 
of November, the sun being then about 16 \ minutes in 
advance of the clock. Mean and apparent time coincide 

278. What is the equation of time ? How caused ? 279. When is it greatest ? 
When nothing ? 



174 TIME. 

four times a year, namely: April 15th, June 15th, Sep- 
tember 1st, and December 24th. The equation of time 
then becomes nothing. 

280. A Sidereal Year is the period of time that elapse.! 
from the sun's leaving any star until it returns to the 
same again. 

This is the true period of the annual revolution of the earth, and 
is equal to 365 days, 6 hours, 9 minutes, 9 seconds. Owing, how- 
ever, to the precession of the equinoxes, the sun advances through 
all the signs, from either equinox to the same again, in a shorter 
period. 

281. A Tropical Year is the period that elapses from 
the sun's leaving the vernal equinox until it arrives at 
it again. It is 20 min. 20 sec. shorter than the sidereal 
year. 

Its length is, therefore, 365d. 5h. 48m. 49s. which is the civil 
year, or the year of the calendar, deducting the 5h. 48m. 49s. ; and 
as this is very nearly one-fourth of a day, one day is added every 
fourth year, which makes what is called leap year, or bissextile. 
The tropical year is sometimes called an equinoctial or solar year. 

280. What is a sidereal year ? 281. A tropical year ? Its length ? 



— - — 



SECTION XII. 



REFRACTION. 



232. The observed places of the heavenly bodies are 
not always the true places ; for the rays of light when 
passing obliquely through the earth's atmosphere undergo 
a change of direction, called Refraction. 

283. It is a general fact that tli3 rays of light when 
passing obliquely from one medium into another of a 
different density, are turned from their course and made 
to pass more obliquely if the medium which they enter 
is rarer, and less obliquely if it is denser than that which 
they leave. Thus, in passing from air into water, or 
from water into glass, the direction 
would be less oblique ; but in pass- 
ing from water into air, more oblique. 

Suppose n m to represent the surface of 
water, and S O a ray of light, entering the 
water at O. Instead of keeping on in the 
direction S O, it is bent toward the per- 
pendicular A B, and thus passes less obliquely. 

284. As the earth's atmosphere is not of uniform 
density, but grows more and more dense toward the sur- 

283. Why are not the ohserved places of the heavenly hodies always the true 
places ? What is refraction ? 283. What is the general fact ? 284. How are the 
rays of light Dent in passing through the atmosphere ? Explain from the diagram. 




176 



BE FRA CTI OX. 



face of the earth, the rays of light which proceed from 
any body are constantly bent more and more toward a 
perpendicular direction ; and since we see an object in 
the direction in which the ray of light strikes the eye, the 
apparent altitude of the body will be increased. 

Suppose E to represent 
the earth, and A B C D, 
portions or strata of the 
atmosphere of different den- 
sities, P the place of ob- 
servation. Suppose a ray 
of light from the star S 
strike the atmosphere at a ; 
on account of refraction, 
instead of proceeding in 
the direction S A, it de- 
scribes a &, b c, and c P, 
reaching the spectator at 
P, and in the direction of c P ; so that the star appears in that 
direction at S', and is thus elevated above its true position at S. 
As the atmosphere does not consist of distinct strata, as represented, 
but diminishes uniformly in density from the surface of the earth, 
the broken line a h c P is in reality a curve, and the line S' P a 
tangent to it at the point P. 

285. The effect of refraction is greatest upon a body 
. when it is in the horizon, and diminishes toward the 

zenith, where it is nothing. At the horizon, it amounts 
to about 33 minutes. 

286. There is no refraction at the zenith, because M 
that point every ray of light strikes the atmosphere pcr- 




a B c 



285. When is the refraction greatest ? When is it nothing? What is its amount 
at the horizon ? 286. Why is there no refraction at the zenith ? 



REFRACTION. 177 

pendicularly, and refraction only takes place when the 
direction of the rajs is oblique ; at the horizon, they are 
more oblique than they can be at any point above it ; 
hence the refraction is greatest there. 

287. At the horizon, the amount of refraction is some- 
what greater than the apparent diameter of the sun or 
moon ; and hence these bodies appear to be above the 
horizon when they are actually below it. 

288. The times of the rising of all the heavenly bodies 
are, therefore, accelerated, and those of their setting 
retarded, by refraction ; each one appears to be above 
the horizon before it has actually risen, and is seen above 
the horizon after it has actually set. 

Eefraction very rapidly diminishes from the horizon towards the 
zenith. At the horizon its mean value is 3£'; at 10° of altitude, 
15i'; at 30°, 1£'; at 45°, 57"; at 80°, 10"; at 90°, 0. 

288. What is the effect of refraction upon the sun and moon at rising and setting ? 
289. Upon the rising and setting of other bodies ? 



23 



^mmr 






SECTION XIII. 

PARALLAX. 

289. The apparent position of a heavenly body is also 
affected by a change of place of the observer. Thus, the 
moon if viewed from one point of the earth's surface 
appears in a somewhat different position than it would 
if seen from another point at a considerable distance 
from the first ; its true position being that at which it 
would appear if viewed from the centre of the earth. 
This gives rise to what is called .Parallax. 

290. Parallax may be defined as the difference between 
the observed place of a heavenly body and its position if 
viewed from the centre of the earth or the centre of its 
orbit. Hence parallax is either Diurnal or Annual. 

291. The Diurnal Parallax of a body is the differ- 
ence between its altitude as seen at the surface of the 
earth and that which it would have if viewed from the 
centre. 

In the diagram, let the small circle represent the earth, having itf 
centre at E; A, B, and C, a body as seen at different altitudes 
from the place P; EH, the plane of the rational horizon; Pli, 
the plane of the sensible horizon, and E Z, the direction of the 



239. What else affects the r.pparent place of the heavenly hodics ? What is the 
change called ? 200. How may parallax be defined ? Of how may kinds is it .' 
291. What is diurnal parallax ? 







— — 



PARALLAX. 



179 




zenith. At A, the body being in the sensible horizon, its apparent 
altitude will be nothing ; but if viewed from E, it would appear 
to be above the horizon a 
distance equal to the an- 
gle m E H, or its equal 
m A h, since the differ- 
ence in direction between 
the lines E H or P h, and 
E in, is the difference be- 
tween the apparent and 
true altitude. At B, there 
is evidently a less differ- 
ence of direction between 
the lines P n and E o, and 
when the body is at C, the 
centre of the earth, the place of the observer, and the j^osition of 
the body being all on the same straight line, the true is the same 
as the apparent altitude. 

292. It is evident that the apparent altitude is always 
less than the trne altitude, except when the body is seen 
in the zenith; and that there is the greatest differ- 
ence when the body is in the horizon. The effect 
of parallax is, therefore, to diminish the altitude of a 
body. 

293. The parallax of a body is greatest when it is in 
the horizon, and diminishes towards the zenith, where it 
is nothing. The parallax of a body when in the horizon 
is called its Horizontal Parallax. 

In the preceding diagram, the angle m A h, or its equal P A E, 
is called the angle of parallax; o B n, or P B E, is the angle of 



292. How does it affect the altitude ! 
nothing ? 



298. WTaen is the parallax greatest ? When 



180 PARALLAX. 

parallax for the position B. The angular distance of the sensible 
and rational horizons is, of course, the horizontal parallax. 

294. The greater the distance of a body from the 
earth, the smaller is the angle of parallax. Thus, the 
horizontal parallax of the moon is nearly 1°, while that 
of the sun is less than 9". When the parallax of a body 
is known, its distance from the earth can be determined. 

That the parallax diminishes as the distance of a body from the 
earth increases will be understood by examining the accompanying 
diagram. The horizontal parallax of a body at A is A E H or 



P A E ; but at B, it is the smaller angle B E II, or P B E. The 
horizontal parallax of any body is really the angle subtended by 
the semi-diameter of the earth at the distance of the body ; and, of 
course, the greater the distance, the smaller the angle. 

295. Annual Parallax is the change which would 
take place in the position of a star, if it could be viewed 
from the centre of the orbit instead of the orbit itself. 

The usual method of finding the parallax of a body by viewing 
it at different parts of the earth's surface is entirely useless in the 
case of the stars, as the displacement thus occasioned in the posi- 
tions of any of them is utterly inappreciable ; the radius of the 
earth at a distance so immense being practically but a mathematical 
point. If, however, we view the same star at intervals of six 
months, our stations of observation will be about 180 millions of 



294. How does it depend upon the distance of a body ? What is the parallax 
of the moon ? Of the sun ? What can he determined hy means of the parallax ? 
235. What is annual parallax ? Why applied to the stars ? 



— 



PARALLAX. 



181 



miles apart ; and the amount of displacement thus occasioned, 
when reduced to the centre of the orbit, is what is meant by the 
annual parallax. 

296- The greatest parallax jet discovered in the case 
of any star is somewhat less than 1", so that the earth's 
orbit is but little more than a mere point at the nearest 
star. The parallaxes of twelve stars have been deter- 
mined with considerable precision, the smallest being 
less than 2V °f a single second. 

The following list contains all the stars whose parallax has teen 
found: 



NA3EE. 


PARALLAX. 


NAME. 


PARALLAX. 


a Centauri, 


0.9187 


a Lyrae, 


0.155 


61 Cygni, 


0.5638 


Sirius, 


0.150 


21258 Lalande, 


0.2709 


1 Ursae Majoris, 


0.133 


17415 Oeltzen, 


.247 


Arcturus, 


0.127 


1830 Groombridge, 


.226 


Polaris, 


0.067 


70 Ophiuchi, 


.16 


Capella, 


0.046 



296. What is the greatest parallax of a star ? 
many stars has the parallax heen discovered ? 



The least discovered? Of how 



INDEX OF ASTRONOMICAL TERMS. 



[This index will be useful for Review, as well as Reference. The 
definitions, which are very brief, may also be committed to memory 
in connection with the Text.] 

A'e?*olile. — (Greek, aer, the air, and litlios, a stone.) A meteoric stone. 

Almacan' tavs. — (Arabic.) Parallels of altitude. 

Altitude. — (Latin, altitudo, height.) The distance of a body above 

the horizon. 
Amplitude. — (Latin, amplitudo, largeness.) The distance of the 

sun at its rising or setting from the east or west point of the 

horizon. 
An' nular.— (Latin, cumulus, a ring.) A term applied to an eclipse 

in which the sun's disk looks like a ring. 
A?itarc' tic. — (Greek, anti, against, and arcios, a bear.) The circle 

on the other side of the heavens from the Constellation of 

the Bear. 
Ant ip r odes. — (Greek, anti, against, and podes, feet.) Those inhabi- 
tants of the earth who live on exactly opposite sides of the 

earth, or feet to feet. 
Antosci. — (an-te'si,) (Greek, anti, against, and oikos, a house.) Those 

who live under the same meridian, but on opposite sides of 

the equator and at equal distances from it. 
Aptie' lion. — (Greek, apo or ap7i, from or away, and helios, the sun.) 

The point of the planet's orbit farthest from the sun. 
Ap'ogee.— (Greek, apo and ge, the earth.) The point of the moon's 

orbit farthest from the earth. 
Ap'si-des. — Plural of Apsis. 
Apsis. — (Greek, apsis, a joining.) The apsis line is the line which 

joins the aphelion and perihelion of a planet's orbit. 



IXDEX OF ASTBOXOMICAL TERMS. 188 

■Arc? tic. — (Greek, arktos, a bear.) Xear the Constellation of the Bear. 
sis'teroid. — (Greek, aster, a star, and oid, like.) A small planet 

which resembles in appearance a faint star. 
Astronomy. — (Greek, aster, a star, and nomas, a law.) The science 

of the heavenly bodies. 
citmo sphere. — (Greek, atmos. vapor, and sphmra, a sphere.) The 

body of air, vapor, etc., which encompasses the earth and 

some of the other planets. 
Axis, plural sixes. — (Latin, axis, an axle.) The imaginary line on 

which the earth turns. 
Az' imuth. — (Arabic.) The distance of a body from the north or 

south point of the horizon. 

Si' nary. — (Latin, oini, two by two.) A term applied to systems of 
double stars. 

Card ' inal. — (Latin, cardo, a hinge.) The term applied to the four 
principal points of the horizon. 

Ce?itrif r ugal. — (Latin, centrum, the centre, and fugio, to flee.) 
The centrifugal force is that by which a body recedes from 
the centre of motion. 

Ce?itrip r etal. — (Latin, centrum, and r peto, to seek.) The centripetal 
force is that by which a body is drawn toward the centre of 
motion. 

Comet. — (Latin, coma, hair.) A comet is so called because it is sur- 
rounded by a nebulous appearance resembling hair. 

Co?icentric. — (Latin, con, together, and centrum) Applied to circles 
drawn around the same centre. 

Conjunction. — (Latin, con, together, and jungo, to join.) The 
apparent meeting of a planet with the sun. 

Constellatio?i. — (Latin, con, and stella, a star.) A group of stars. 

Cra'ter. — (Latin, crater, a cup.) A term applied to some of the 
mountains in the moon. 

Crepus' culum . — (Latin.) Twilight. 

Culminate. — (Latin, culmen, the top.) To pass the meridian, be- 
cause then a body arrives at its greatest altitude. 

^Diameter. — (Greek, dia, through, and metron, a measure.) The line 
which measures through a circle or sphere. 



184 INDEX OF ASTRONOMICAL TERMS. 

jDigit. — (Latin, digitus, a finger.) One of the twelve equal divisions 

of the diameter of the sun's or moon's disk. 
3)isk. — (Latin, discus, a quoit.) The circular illuminated surface 

presented by a heavenly body. 

Eccentricity. — (Greek, ec, from, and centron, a centre.) Distance 

or departure from the centre. 
Eclipse. — (Greek, ecleipsis, a fainting away.) The obscuration of 

the disk of the sun or moon. 
Ecliptic. — (From eclipse.) The great circle in which the sun appears 

to move ; so called, because eclipses only take place when the 

moon is in or near its plane. 
Elongation. — (Latin, e, out, and longus, long.) The apparent 

departure of a planet from the sun. 
Equator. — (Latin, equo, to divide equally.) The great circle which 

divides the earth into northern and southern hemispheres. 
Equinoctial. — (Latin, equus, equal, and nodes, nights.) A great 

circle in the heavens ; so called, because, when the sun is in 

it, every place on the earth's surface has equal days and nights. 

Eocies, plural Eoci. — (Latin, focus, a fire-place.) Applied to the 
two points round which an ellipse is drawn. The sun is 
situated at the lower focus of every planetary orbit. 

Gal'axy. — (Greek, galaxias, the milky-way.) The cloudy zone which 

encompasses the heavens, consisting of vast numbers of stars. 

(Latin, Via Lactea) 
Geoce?i'tric. — (Greek, ge, the earth, and centron, a centre.) Seen 

from the earth as a centre. 
Gib'boits. — (Latin, gibbus, convex.) Applied to the partial disk of 

the moon when more than half is visible. 

Eel ioce?i' trie. — (Greek, hclios, the sun, and centron, the centre.) 

Seen from the sun as a centre. 
Eori/zon. — (Greek, horizo, to bound.) The circle in the heavens 

which bounds our view. 

Ztibration. — (Latin, libratio, a balancing.) An apparent vibratory mo- 
tion of the moon from side to side, or pole to pole, by which we 
are enabled to see glimpses of the hemisphere turned from us. 

ZfUJlCtr. — (Latin, I una, the moon.) Pertaining to the moon. 



IXDEX OF ASTRONOMICAL TERMS. 185 

Meridian. — (Latin, meridies, mid-day.) The circle at which the 

sun arrives at noon each day. 
Jfe'teor. — (Greek, meteora, things in the air.) A small luminous 

body which shoots like a star from the sky. 

Na'dir. — (Arabic, nazeer, opposite.) The point opposite the zenith. 

JVeb'ula. — (Latin, nebula, a little cloud.) A faintly luminous appear- 
ance in the heavens resembling a speck of cloud or mist. 
(Plural nebula) 

Node. — (Latin, nodus, a knot.) The nodes are the points at which 
a planet's orbit crosses the plane of the ecliptic. 

JVa' 'cleus : — (Latin, nucleus, a kernel.) The bright and seemingly 
solid part of a comet. 

Natation. — (Latin, nutalio, a nodding.) A vibratory motion of the 
earth's axis. 

Occultation. — (Latin, occultatio, a hiding.) The concealment of 
a star or planet by the interposition of the moon. Applied 
also to Jupiter's satellites when concealed by their primary. 

Octant. — (Latin, octo, eight.) The eighth part of a circle. 

Orbit. — (Latin, orbis, a circle.) The path of a heavenly body. 

^Par'allax.— {Greek, parallaxis, change.) The difference in the 

apparent position of a heavenly body, occasioned by a change 

of place in the observer. 
!Penum'dra. — (Latin, pene, almost, and umbra, a shadow.) The 

partial interception of the sun's rays on each side of the 

umbra, or total shadow of the earth or moon. 
'Per / igee. — (Greek, peri, near, and ge, the earth.) The point of the 

moon's orbit nearest the earth. 
'Perihe'lion. — (Greek, peri, near, and helios, the sun.) The point 

of a planet's orbit nearest the sun. 
IPerios'ci. — (Greek, peri, around, and oikeo, to dwell.) Those who 

dwell under the same parallel of latitude, but under opposite 

meridians ; so that when it is day to one, it is night to the 

other. 
*Phase. — (Greek, phasis, an appearance.) The portion of a heavenly 

body's disk visible at any time. 
*Planet. — (Greek, planetes, a wanderer.) The planets are so called 

because they constantly change their places among the stars. 
24 



180 INDEX OF ASTRONOMICAL TERMS. 

Precession. — (Latin, pre, before, and cessio, a going.) Applied to 
the shifting of the equinoxes because they go, as it were, to 
meet the sun. 

Quadrant* — (Latin, quadrans, the fourth part.) The fourth part of 

a circle. 
Quadrature* — (Latin, quadra, a square.) The position of a planet 

when it is 90° from the sun. 
Qua?'' tile.— (Lnim, quartus, fourth.) The aspect of two planets 9(T 

from each other. 

'Radius. — (Latin, radius, a ray.) Plural, radii. Lines drawn from 

the centre of a circle to the circumference, as rays proceed 

from the sun. 
'Padius-vec' tor. — (Latin, radius, and rector, that which carries.) 

The line which connects the sun and a planet, and which 

may be conceived as carrying the latter, as it sweeps over the 

plane of the orbit. 
jRefraction. — (Latin, refractio, a breaking.) The deviation of a ray 

of light from a straight line. 
/ Ret?'og?'ade. — (Latin, retro, backward, and gradior, to go.) An 

apparent backward motion of the planets ; that is, a motion 

from east to west. 

Satellite. — (Latin, satelles, a guard.) An attendant body of a pri- 
mary planet. 
Secondary. — (Latin, sec unci us, second.) Applied to planets revolving 

around other planets ; also, to great circles perpendicular to 

other great circles. 
Sextant. — (Latin, sextus, sixth.) The sixth part of a circle. 
Sex tile. — (Latin, sextilis, the sixth.) The aspect of two planets 60 

degrees, or the sixth of a circle, from each other. 
Side' real. — (Latin, sidus, a star.) Pertaining to the stars. 
Sola?'. — (Latin, sol, the sun.) Pertaining to the sim. 
Solstice. — (Latin, sol, and sto, to stand.) The point of the ecliptic 

at which the sun appears to stand in respect to declination 

and meridian altitude. 
Spheroid, — (Greek, sphaira, a sphere, and oid, like.) A solid that 

resembles a sphere. 
Stellar. — (Latin, Stella, a star.) Pertaining to the stars. 



IXDEX OF ASTRONOMICAL TERMS. 187 

Sy)iod' ical. — (Greek, syn. together, and odos, a pathway.) Applied 
to the interval between two successive conjunctions of a planet. 

Syz'j'gies. — (Greek, syzygia, a conjunction.) A term applied to the 
conjunction and opposition of the moon. 

'Telescope. — (Greek, tele, at a distance, and scopeo, to see.) An 
instrument for viewing objects at a distance. 

terminator, — (Latin, terminus, a boundary.) The line which di- 
vides the enlightened from the dark part of the moon. 

Transit. — (Latin, transitus, a passage across.) The passage of an 
inferior planet across the sun's disk. Applied also to the 
passage of Jupiter's satellites across the disk of the primary. 

Trine. — (Latin, trinus, three.) The aspect of two planets 120° from 
each other. 

'2'rojjic.— (Greek, trope, turning.) Applied to the two circles which 
limit the sun's declination, because when it arrives at one it 
turns and goes back to the other. 

Umbra. — (Latin, unibra, a shadow.) The conical shadow of the 
earth or moon. Applied also to the dark part of a solar spot. 

Vertical. — (Latin, vertex, the point about which anything turns; 
hence, the top.) Applied to great circles which pass through 
the zenith, or overhead. 

Ze'nith. — (Arabic.) The point overhead. 

Zo'diac. — (Greek, zodiakos, pertaining to animals.) The belt which 
contains the twelve constellations of the ecliptic, represented 
by animals ; as, Aries, the ram ; Taurus, the bull, etc. 

Zone. — (Greek, zone, a girdle.) A division of the earth's surface. 



APPENDIX. 



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APPENDIX. 



189 



TABLE II. -ASTEROIDS, OR MINOR PLANETS. 





1801 


. 2 Pallas 


1802 
1804 


4 Vesta 


1807 


5 Astraea 


1845 


6 Hebe 


1847 


7 Iris 


1847 


8 Flora 


1847 


9 Metis 


1848 


10 Hygeia 


1849 


11 Parthenope 

12 Victoria 


1850 
1850 


13 Egeria 


1850 




1851 




1851 


16 Psyche 


1852 


17 Thetis 

18 Melpomene 

19 Fortuna 


1852 
1852 
1852 
1852 
1852 
1852 
1852 
1853 
1853 
1853 
1853 
1854 
1854 
1854 
1854 
1854 
1854 
1855 
1855 
1855 
1855 
1856 
1856 
1856 
1856 


20 Massilia 


21 Lutetia 




23 Thalia 

24 Themis 


25 Phocea 


26 Proserpine 

27 Euterpe 


28 Bellona 


29 Amphitrite 


31 Euphrosyne 

32 Pomona 


83 Polyhymnia 

31 Circe 

35 Leucothea 

36 Atalanta 


37 Fides 


38 Leda 


39 Lgetitia 


40 Harmonia 

41 Daphne 



Isis 

Ariadne. . . 

Nysa 

Eugenia. . . 
Hestia. . . . 
Melete. . . . 
Aglaia .... 

Doris 

Pales .... 
Virginia . . 
Nemausa. . 
Europa. . . . 
Calypso. . . 
Alexandra . 
Pandora.. 



DATK op 
DISCOVEBY. 



1856 

1857 
1857 
1857 
1857 
1857 
1857 
1857 
1857 
1857 
1858 
1858 
1858 
1858 
1858 



Mnemosyne | 1859 



Concordia . 

Danae 

Olympia 

Erato 

Echo 

Ausonia 

Angelina. . . 

Cybele 

Maia 

Asia 

Hesperia . . . 

Leto 

Panopea 

Feronia 

Niobe 

Clytie 

Galatea 

Eurydice 

Freia 

Frigga 

Diana 

Eurynome . . 

Sappho 

Terpsichore. . 
Alcmene 



1860 
1860 
1860 
1860 
1860 
186; 



188: 

186 

186: 

186: 

186: 

186 

186: 

1862 

1862 

1862 

1862 

1862 

1863 

1863 

1864 

1864 

1864 



190 



APPENDIX. 
TABLE 11.— Continued. 



NAME. 


1 

DATE OF 
DISCOVERY. 


NAME. 


DATE OP 
DISCOVERY. 


83 Beatrix 


1865 
1865 


! 98 Iantlie 


1868 


84 Clio , 


99 Dike 


1868 


85 Io 


1865 


100 Hecate. . . 


1868 

ISfift 


86 Semele 


1866 
1866 
1866 
1866 
1866 
1866 


101 Helena 


87 Sylvia 


\ 102 Miriam 


88 Thisbe 


103 Hera 


1868 
1868 
1868 
1868 


89 Julia 


104 Clvmene 


90 Antiope 




91 iEgina 


• 106 Dione 


92 Undina 

93 Minerva 

94 Aurora 

95 Arethusa 


1867 
1867 
1867 
1867 


107 Camilla 

108 Hecuba 

109 Felicitas 

110 Lvdia 


1869 
1869 
1869 
1869 


96 Mg\e 


1868 
1868 


Ill Iphigenia 

112 


1870 


97 Clotho 


1870 







^ s !.» 




